The divorce, on paper, sounds common enough: the years go by, the kids grow up, the couple drifts apart. The marriage, they finally tell a court, has become “irretrievably broken.”

But that’s where the familiar outlines of many a breakup end, because this one concerns Bill Gates and Melinda French Gates.

In financial terms, this is a split for the ages. Untangling their combined fortune after 27 years of marriage is a colossal undertaking. There are the mansions and the private jets. Vast tracts of land and a hotel brand. Huge investments in public corporations and private businesses.

And, of course, the Bill & Melinda Gates Foundation—the largest philanthropic organization of its type on the planet.

“They’re probably about the biggest divorce imaginable,” said Janet George, a family law attorney at McKinley Irvin.

The work has already begun, starting with a separation contract, according to the petition for divorce French Gates, 56, filed in their longtime home state of Washington.

She signed the papers from Bellevue, where they have a sprawling $130 million lakefront mansion. Bill Gates, 65, signed from Palm Desert, in southern California, where they own another home.

Representatives for the pair declined to comment on the divorce and whether they had a pre-nuptial agreement.

But a pre-nup, if there is one, wouldn’t matter, because the separation contract supersedes any previous agreement.

There have already been more than $3 billion of transfers from Cascade Investment, also known as BMGI (Bill and Melinda Gates Investments), to French Gates, a sliver of their $145 billion fortune at the time of the divorce announcement.

So far, there’s no clear pattern to how the shares are being divided.

The 2.25 million shares of Deere & Co. French Gates received are worth about $800 million, but they made up just 7% of the ex-couple’s total stake in the company. Meanwhile, she received all the firm’s shares in Coca-Cola Femsa, a Mexican distributor, worth around $130 million.

It may never be made public what happens with their collection of private companies, such as the Four Seasons brand in which they bought a 47.5% stake in back in 2007 along with Saudi Arabian Prince Al Waleed bin Talal Al Saud. Or their property investments, which make them the largest private farmland owner in the U.S.

Though not part of their fortune, one of the couple’s biggest assets is their foundation, which has $50 billion in assets, more than 1,600 employees and offices globally.

Their philanthropy has particularly transformed Bill’s reputation from the monopolistic capitalist behind Microsoft Corp. to techno-geek do-gooder tackling global hunger and health.

The divorce has already cost Gates some of that image, with reports of infidelity, his ties to the late pedophile Jeffrey Epstein and coverage that his money manager Michael Larson operated a toxic work environment.

Gates and French Gates have established their own philanthropic arms outside of the foundation and their interests have diverged in recent years—Bill on climate change, Melinda on gender equality. For the career philanthropists they’ve become, that means how the fortune is split will determine where funding flows to these causes for decades.