Allowance for loan losses - Activity in Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Financing Receivable, Allowance for Credit Losses
Balance at beginning of period	$ 543,666	$ 643,018	$ 569,348	$ 623,426
Provision (reversal) for loan losses	36,539	54,387	118,555	185,504
Charge-offs	(92,518)	(83,055)	(240,347)	(224,512)
Balance at end of period	512,365	633,718	512,365	633,718
ASC Subtopic 310-30
Financing Receivable, Allowance for Credit Losses
Balance at beginning of period	120,818	156,328	122,135	119,505
Provision (reversal) for loan losses	(14,617)	17,854	(2,007)	78,317
Charge-offs	(11,591)	(5,623)	(25,518)	(29,263)
Balance at end of period	$ 94,610	$ 168,559	$ 94,610	$ 168,559