Transfers of financial assets and servicing assets - Initial fair value of assets obtained as proceeds from residential mortgage loans securitized (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 122,874	$ 138,659	$ 337,140	$ 399,278
GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	88,000	110,000	247,000	320,000
FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	33,000	26,000	84,000	72,000
Trading account debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	120,658	133,536	331,112	382,622
Trading account debt securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	88,139	109,911	247,091	319,769
Trading account debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	32,519	23,625	84,021	62,853
Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,216	2,625	6,028	7,198
Debt securities available-for-sale
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		2,498		9,458
Debt securities available-for-sale | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		2,498		9,458
Level 1
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 1 | Trading account debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 1 | Trading account debt securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 1 | Trading account debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 1 | Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 1 | Debt securities available-for-sale
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		0		0
Level 1 | Debt securities available-for-sale | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		0		0
Level 2
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	120,658	136,034	331,112	392,080
Level 2 | Trading account debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	120,658	133,536	331,112	382,622
Level 2 | Trading account debt securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	88,139	109,911	247,091	319,769
Level 2 | Trading account debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	32,519	23,625	84,021	62,853
Level 2 | Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 2 | Debt securities available-for-sale
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		2,498		9,458
Level 2 | Debt securities available-for-sale | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		2,498		9,458
Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,216	2,625	6,028	7,198
Level 3 | Trading account debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 3 | Trading account debt securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 3 | Trading account debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 3 | Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 2,216	2,625	$ 6,028	7,198
Level 3 | Debt securities available-for-sale
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		0		0
Level 3 | Debt securities available-for-sale | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		$ 0		$ 0