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		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 118,256	$ 195,665	$ 263,367	$ 386,742
Level 1
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 2
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	116,296	192,785	258,998	381,053
Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,960	2,880	4,369	5,689
Trading account debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	116,296	192,785	258,998	381,053
Trading account debt securities | Level 1
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Trading account debt securities | Level 2
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	116,296	192,785	258,998	381,053
Trading account debt securities | Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Trading account debt securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	77,269	106,729	155,163	208,717
Trading account debt securities | GNMA | Level 1
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Trading account debt securities | GNMA | Level 2
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	77,269	106,729	155,163	208,717
Trading account debt securities | GNMA | Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Trading account debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	37,640	72,555	95,330	158,835
Trading account debt securities | FNMA | Level 1
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Trading account debt securities | FNMA | Level 2
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	37,640	72,555	95,330	158,835
Trading account debt securities | FNMA | Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Trading account debt securities | FHLMC
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,387	13,501	8,505	13,501
Trading account debt securities | FHLMC | Level 1
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Trading account debt securities | FHLMC | Level 2
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,387	13,501	8,505	13,501
Trading account debt securities | FHLMC | Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,960	2,880	4,369	5,689
Mortgage Servicing Rights | Level 1
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Mortgage Servicing Rights | Level 2
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Mortgage Servicing Rights | Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 1,960	$ 2,880	$ 4,369	$ 5,689