Warren Buffett's Berkshire Hathaway bought 38.6% of the Pilot Flying J truck-stop chain on Tuesday, and a firm you've probably never heard of played a key role.

BDT & Company advised Pilot Flying J on the deal with its in-house finance arm, BDT Capital Partners, exiting its minority investment in the company.

Not heard of BDT & Company? That's by design. Despite having more than $12 billion in assets under management, the firm doesn't even have a public-facing website.

It does, however, have more than 150 employees spread out over four offices in Chicago, New York, London, and Frankfurt, Germany, who advise extremely wealthy clients with family-owned businesses.