Servicing rights - Residential mortgage servicing rights (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Servicing rights
Unpaid Principal Amount	$ 1,676,150	$ 1,470,532
Residential
Servicing rights
Unpaid Principal Amount	7,982,024	7,466,679
Fair Value	84,638	93,065	$ 85,658	$ 91,708	$ 85,554	$ 72,295
Possible impact of adverse changes to key assumptions
Prepayment rate (10% adverse change)	(3,880)	(3,281)
Prepayment rate (20% adverse change)	(7,451)	(6,330)
Discount rate (10% adverse change)	(3,153)	(3,716)
Discount rate (20% adverse change)	(6,082)	(7,159)
Cost of servicing (10% adverse change)	(1,570)	(1,683)
Cost of servicing (20% adverse change)	(3,140)	(3,365)
Fannie Mae | Residential
Servicing rights
Unpaid Principal Amount	3,245,359	2,848,435
Fair Value	33,414	34,562
Ginnie Mae | Residential
Servicing rights
Unpaid Principal Amount	2,453,034	2,350,301
Fair Value	28,125	29,586
Freddie Mac
Servicing rights
Unpaid Principal Amount	1,120,377	964,377
Possible impact of adverse changes to key assumptions
Prepayment rate (10% adverse change)	(117)	(63)
Prepayment rate (20% adverse change)	(231)	(124)
Discount rate (10% adverse change)	(328)	(490)
Discount rate (20% adverse change)	(645)	(791)
Freddie Mac | Residential
Servicing rights
Unpaid Principal Amount	2,283,631	2,267,943
Fair Value	$ 23,099	$ 28,917
Minimum | Residential
Servicing rights, valuation assumptions
Forward prepayment assumptions	7.30%	6.10%
Discount rate	9.00%	9.00%
Servicing expense	$ 70	$ 70
Minimum | Freddie Mac
Servicing rights, valuation assumptions
Forward prepayment assumptions	0.10%	0.00%
Discount rate	6.00%	12.00%
Servicing expense (as a percent)	0.20%	0.20%
Maximum | Residential
Servicing rights, valuation assumptions
Forward prepayment assumptions	18.70%	13.90%
Discount rate	11.00%	11.10%
Servicing expense	$ 85	$ 85
Maximum | Freddie Mac
Servicing rights, valuation assumptions
Forward prepayment assumptions	35.40%	35.00%
Discount rate	6.00%	12.00%
Servicing expense (as a percent)	0.30%	0.20%
Weighted Average | Residential
Servicing rights, valuation assumptions
Forward prepayment assumptions	10.60%	8.40%
Discount rate	9.60%	9.70%
Servicing expense	$ 75	$ 75
Weighted Average | Freddie Mac
Servicing rights, valuation assumptions
Forward prepayment assumptions	5.30%	5.00%
Discount rate	6.00%	12.00%
Servicing expense (as a percent)	0.20%	0.20%