Allowance for credit losses - Activity in Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Financing Receivable, Allowance for Credit Losses
Balance at beginning of period	$ 477,708	$ 569,348
Provision (reversal) for loan losses	189,731	41,825
Balance at end of period	919,716	$ 550,628
ASC Subtopic 310-30
Financing Receivable, Allowance for Credit Losses
Balance at beginning of period	122,135
Provision (reversal) for loan losses	7,726
Net charge-offs	(5,714)
Balance at end of period	$ 124,147