Transfers of financial assets and servicing assets - Initial fair value of assets obtained as proceeds from residential mortgage loans securitized (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 13,759,000	$ 118,256,000	$ 25,003,000	$ 263,367,000
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	13,393,000	116,296,000	24,359,000	258,998,000
Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	366,000	1,960,000	644,000	4,369,000
Trading account debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	13,393,000	116,296,000	24,359,000	258,998,000
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	13,393,000	116,296,000	24,359,000	258,998,000
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		77,269,000	1,067,000	155,163,000
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
Trading account debt securities | GNMA | Level 2
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		77,269,000	1,067,000	155,163,000
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
Trading account debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	13,393,000	37,640,000	23,292,000	95,330,000
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	13,393,000	37,640,000	23,292,000	95,330,000
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	0	1,387,000	0	8,505,000
Trading account debt securities | FHLMC | Level 1
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		0		0
Trading account debt securities | FHLMC | Level 2
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		1,387,000		8,505,000
Trading account debt securities | FHLMC | Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		0		0
Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	366,000	1,960,000	644,000	4,369,000
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	$ 366,000	$ 1,960,000	$ 644,000	$ 4,369,000