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		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 11,900,000	$ 36,684,000	$ 36,903,000	$ 300,051,000
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	11,599,000	35,875,000	35,958,000	294,872,000
Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	301,000	809,000	945,000	5,179,000
Trading account debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	11,599,000	35,875,000	35,958,000	294,872,000
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	11,599,000	35,875,000	35,958,000	294,872,000
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	1,421,000	14,190,000	2,488,000	169,352,000
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	0
Trading account debt securities | GNMA | Level 2
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,421,000	14,190,000	2,488,000	169,352,000
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	0
Trading account debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	10,178,000	21,685,000	33,470,000	117,015,000
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	10,178,000	21,685,000	33,470,000	117,015,000
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		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
Trading account debt securities | FHLMC | Level 2
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized				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
Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	301,000	809,000	945,000	5,179,000
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	$ 301,000	$ 809,000	$ 945,000	$ 5,179,000