"State Farm, as an Illinois-based company, we are interpreting this law in a way that we feel is going to be fair and consistent for all of our associates," Supple said.

The benefits expansion includes medical, dental, vision, spouse life, spouse voluntary accidental death and dismemberment, and long-term care insurance. Once enrolled and accepted, the benefits begin immediately.

The cost for State Farm to cover the new partners was not available, but Supple said "we don't see the volume of participation or the level of claims to be a significant cost factor." Earlier this year, State Farm announced a plan to limit its exposure to rising health care expenses among its retirees, starting in 2012.

State Farm did not offer domestic partner health benefits previously, unlike the 57 percent of Fortune 500 companies in 2010 that did, according to the Human Rights Campaign, a civil rights group. Energy giant Exelon and fellow insurer Allstate, both based in Illinois, offer such benefits. Peoria-based Caterpillar does not.

"Everybody moves along at their own pace," Supple said. "A lot of them do offer it, a lot of them do not."