Billionaire investor Warren Buffett had an open marriage with his first wife, Susan Thompson, until her death in 2004.

Thompson left Buffett to pursue a singing career, but they remained amicably married and she gave him permission to date other women.

Thompson also introduced her husband to Astrid Menks, who became his companion. The trio became close friends and even sent out Christmas cards together.

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Warren Buffett started dating his second wife, Astrid Menks, while he was still married to his first wife — and neither of them minded.

The "Oracle of Omaha" spent years in an unconventional marriage until his first wife Susan died in 2004. Family members said the unusual arrangement worked for all those involved. The trio would even send out Christmas cards together — signed Warren, Susan, and Astrid — according to The Daily Mail.

"Unconventional is not a bad thing," Buffett's daughter, Susie Buffett, told The New York Times. "More people should have unconventional marriages."

Here's a look inside Warren Buffett's married life.