UPDATE: The Senate unveiled its full tax plan late Thursday night. Vox’s Dylan Matthews explained the proposal here.

The Senate has a tax reform plan — sort of.

On Thursday, as House Republicans continue to debate their tax reform bill in committee, top Senate Republicans briefed the GOP conference on their own plans for tax reform: a proposal that briefly delays corporate tax rate, completely eliminates the state and local tax deduction, and implements full and immediate expensing on company investments.

Based on conversations with senators in the hallways of Congress after their briefing, we are seeing the broad contours of a tax reform plan that looks substantially different from what the House has proposed, signaling some major obstacles to come between chambers. The House is scheduled to vote next week.

But this is only the opening bid from the Senate — and leadership has indicated it’s open to negotiation.

“There are two or three things in [this proposal] I hate, but I think they have done a good job at focusing on the single three-word goal of raising family income,” Sen. Lamar Alexander (R-TN) told reporters.

Senate rules dictates the tax bill can only increase the deficit by $1.5 trillion in the first 10 years and cannot affect it after that. That rule has already posed a major math problem for Republicans in the House, who are unified in their goal to cut taxes across the board but have faced deep internal disagreement on how to offset those cuts with changes to deductions, loopholes, and credits elsewhere.

It’s not clear how that debate will unfold in the Senate.

Republicans, frantically hoping to pass tax reform by Christmas — for fear that their first year controlling both Congress and the White House will come to an end with no major legislative victories — have spent months debating behind closed doors and avoiding the tough decisions.

There’s no legislative text yet in the upper chamber, and Senate leaders have said there won’t be until the House wraps up debate on their bill. That means the conversation is still conceptual. Here’s what we know — and don’t know:

What we know about the Senate’s tax bill

Senators detailed some of the major provisions of their forthcoming tax plan, and a more detailed outline was released later Thursday.

The working Senate draft would:

Cut the corporate tax rate from 35 percent to 20 percent. But the cut would take effect in 2019, not 2018, as it would in the House bill. President Donald Trump supports cutting the corporate rate in 2018.

Keep seven tax brackets for individual income taxes. The House bill had reduced it to four brackets.

Lower the top individual tax rate from 39.6 percent to 38.6 percent.

Nearly double the standard deduction for individual taxpayers, as the House bill does.

Repeal the alternative minimum tax, which increases taxes for certain affluent or upper-middle-class households, as the House bill did.

Doubles the exemption for estate taxes, up to more than $10 million, but does not fully repeal the tax on inheritances as the House bill eventually would.

Fully repeal the deduction for state and local taxes. The House bill would have kept a deduction for property, but not income, taxes.

Keep the cap for home mortgage deductions at $1 million. The House bill lowered the cap to $500,000.

Keep the adoption tax credit, which the House bill originally eliminated but later restored.

Keep the medical expense deduction, which the House bill eliminated.

Expand the child tax credit and creates a more refundable tax credit than the House bill did.

Instead of having companies “depreciate” investments by deducting them over several years, companies would be able immediately expense all their investments.

The current Senate draft notably does not repeal Obamacare’s individual mandate. However, conservatives are still pushing for the mandate’s repeal to be added to the upper chamber’s plan.

Repealing the mandate would increase the number of uninsured Americans by 13 million over the next 10 years, according to the Congressional Budget Office. But it would also save the federal government $338 billion over the same period because fewer people would be enrolled in Medicaid and in private coverage on the health care law’s marketplaces.

“I believe we should use this opportunity to repeal the individual mandate and we should use the revenues ... to lower rates for hardworking Americans across the board,” Sen. Ted Cruz (R-TX) told reporters after being briefed on the plan.

What we don’t know

But major features of the Senate bill remain opaque.

We don’t know how much Republicans will adjust the income ranges in these seven tax brackets, though they do promise a “reformed rate structure.”

The Senate will be changing the House’s 70/30 rule for pass-through businesses, owner-operated businesses whose profits are taxed as individual income, according to Sen. Rob Portman — but it’s not clear how. Under the House bill, pass-throughs can choose to have 70 percent of their income taxed at the top individual rate and 30 percent taxed under the new 25 percent small business rate.

Nor do we know how much will it increase the federal deficit. Under the Senate’s budget rules, the legislation can only increase the deficit by $1.5 trillion over 10 years and can’t increase the deficit at all after that. Delaying the corporate tax cut and fully repealing SALT, will definitely give the Senate more wiggle room to retain or boost other deductions and credits.

Whatever the official deficit number, though, Republicans continue to argue the tax cuts will pay for themselves.

“I think this tax bill is going to reduce the size of our deficits going forward,” Sen. Pat Toomey (R-PA), one of the primary architects of the plan, told reporters.

But there is also a broad expectation that neither the House bill that is heading for the floor next week nor the Senate bill detailed Thursday will be the final product. Both chambers are already talking about working their issues out in a conference committee in the coming weeks and months.