When Was The BP Trial?

So BP has set up a $20 billion escrow account to pay out damages to those affected by the oil spill and I am wondering, when was BP's trial?

Normally when an accident occurs (civil liability) or negligence leads to disaster (criminal liability) there is some kind of a trial, where the defendant (BP) faces its accusers/those it harmed (the "people affected by the oil spill"). But it seems in this situation that the guilt has been presumed and the trial has simply been skipped and everyone is already participating in the "divvying up the payout"-phase.

Another question I have-- where'd they come up with $20 billion? Sure, it could be more, but I suppose it could also be less. Either way, I am curious how the government has calculated possible damages when it isn't exactly clear who has been affected and how much, at this point.