Servicing rights - Residential mortgage servicing rights (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Servicing rights
Unpaid Principal Amount	$ 2,145,133	$ 1,735,493
Residential MSRs
Servicing rights
Unpaid Principal Amount	9,528,886	8,164,604
Fair Value	76,840	91,174	$ 93,065
Possible impact of adverse changes to key assumptions
Prepayment rate (10% adverse change)	(5,049)	(4,195)
Prepayment rate (20% adverse change)	(9,701)	(8,091)
Discount rate (10% adverse change)	(2,601)	(3,450)
Discount rate (20% adverse change)	(5,028)	(6,654)
Cost of servicing (10% adverse change)	(1,469)	(1,648)
Cost of servicing (20% adverse change)	(2,938)	(3,297)
Fannie Mae | Residential MSRs
Servicing rights
Unpaid Principal Amount	3,700,450	3,388,630
Fair Value	27,632	37,309
Ginnie Mae | Residential MSRs
Servicing rights
Unpaid Principal Amount	2,757,124	2,504,993
Fair Value	25,899	29,869
Freddie Mac
Servicing rights
Unpaid Principal Amount	1,501,998	1,167,476
Possible impact of adverse changes to key assumptions
Prepayment rate (10% adverse change)	(163)	(285)
Prepayment rate (20% adverse change)	(324)	(558)
Discount rate (10% adverse change)	(678)	(381)
Discount rate (20% adverse change)	(1,324)	(746)
Freddie Mac | Residential MSRs
Servicing rights
Unpaid Principal Amount	3,071,312	2,270,981
Fair Value	$ 23,309	$ 23,996
Minimum | Residential MSRs
Servicing rights, valuation assumptions
Forward prepayment assumptions	12.60%	7.10%
Discount rate	9.10%	9.00%
Servicing expense	$ 70	$ 70
Minimum | Freddie Mac
Servicing rights, valuation assumptions
Forward prepayment assumptions	0.10%	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	31.40%	18.70%
Discount rate	11.70%	11.00%
Servicing expense	$ 85	$ 85
Maximum | Freddie Mac
Servicing rights, valuation assumptions
Forward prepayment assumptions	5.10%	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	14.30%	10.10%
Discount rate	9.80%	9.60%
Servicing expense	$ 74	$ 75
Weighted Average | Freddie Mac
Servicing rights, valuation assumptions
Forward prepayment assumptions	2.40%	6.60%
Discount rate	6.00%	6.00%
Servicing expense (as a percent)	0.20%	0.20%