Despite whatt clueless econo-hacks with long and wordy blog posts will have you believe, the entity that ends up footing the bill for a Grexit is one: the ECB. Goldman explains below, and for those strapped for time, here is the TL/DR version:
“More cynically, if a default of bank liabilities is inevitable, it may deem it better to ensure that domestic claimants on Greek banks switch into hard ‘convertible’ Euro banknotes (or offshore accounts), leaving the residual claimants (the ECB which has provided ELA funding) to take the loss.