Servicing rights - Residential mortgage servicing rights (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Servicing rights
Unpaid Principal Amount	$ 1,776,888	$ 1,735,493
Residential MSRs
Servicing rights
Unpaid Principal Amount	8,326,668	8,164,604
Fair Value	78,631	91,174	$ 88,218	$ 93,065
Possible impact of adverse changes to key assumptions
Prepayment rate (10% adverse change)	(4,245)	(4,195)
Prepayment rate (20% adverse change)	(8,110)	(8,091)
Discount rate (10% adverse change)	(2,849)	(3,450)
Discount rate (20% adverse change)	(5,501)	(6,654)
Cost of servicing (10% adverse change)	(1,562)	(1,648)
Cost of servicing (20% adverse change)	(3,123)	(3,297)
Fannie Mae | Residential MSRs
Servicing rights
Unpaid Principal Amount	3,536,350	3,388,630
Fair Value	33,084	37,309
Ginnie Mae | Residential MSRs
Servicing rights
Unpaid Principal Amount	2,513,271	2,504,993
Fair Value	25,587	29,869
Freddie Mac
Servicing rights
Unpaid Principal Amount	1,193,274	1,167,476
Possible impact of adverse changes to key assumptions
Prepayment rate (10% adverse change)	(287)	(285)
Prepayment rate (20% adverse change)	(568)	(558)
Discount rate (10% adverse change)	(381)	(381)
Discount rate (20% adverse change)	(752)	(746)
Freddie Mac | Residential MSRs
Servicing rights
Unpaid Principal Amount	2,277,047	2,270,981
Fair Value	$ 19,960	$ 23,996
Minimum | Residential MSRs
Servicing rights, valuation assumptions
Forward prepayment assumptions	8.50%	7.10%
Discount rate	9.00%	9.00%
Servicing expense	$ 70	$ 70
Minimum | Freddie Mac
Servicing rights, valuation assumptions
Forward prepayment assumptions	0.50%	0.50%
Discount rate	6.00%	6.00%
Servicing expense (as a percent)	0.20%	0.20%
Maximum | Residential MSRs
Servicing rights, valuation assumptions
Forward prepayment assumptions	18.90%	18.70%
Discount rate	11.00%	11.00%
Servicing expense	$ 85	$ 85
Maximum | Freddie Mac
Servicing rights, valuation assumptions
Forward prepayment assumptions	15.90%	15.90%
Discount rate	6.00%	6.00%
Servicing expense (as a percent)	0.30%	0.30%
Weighted Average | Residential MSRs
Servicing rights, valuation assumptions
Forward prepayment assumptions	11.10%	10.10%
Discount rate	9.60%	9.60%
Servicing expense	$ 75	$ 75
Weighted Average | Freddie Mac
Servicing rights, valuation assumptions
Forward prepayment assumptions	6.70%	6.60%
Discount rate	6.00%	6.00%
Servicing expense (as a percent)	0.20%	0.20%