Allowance for credit losses - Activity in Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Financing Receivable, Allowance for Credit Losses
Balance at beginning of period	$ 919,716	$ 550,628	$ 477,708	$ 569,348
Provision (reversal) for loan losses	62,449	40,191	252,180	82,016
Balance at end of period	$ 918,434	543,666	$ 918,434	543,666
ASC Subtopic 310-30
Financing Receivable, Allowance for Credit Losses
Balance at beginning of period		124,147		122,135
Provision (reversal) for loan losses		4,884		12,610
Net charge-offs		(8,213)		(13,927)
Balance at end of period		$ 120,818		$ 120,818