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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 167,879	$ 155,703	$ 554,621	$ 311,836
Trading account debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	165,282	153,966	546,335	307,512
Trading account debt securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	84,896	99,576	293,613	213,608
Trading account debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	76,118	54,390	234,953	93,904
Trading account debt securities | FHLMC
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	4,268		17,769
Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,597	1,737	8,286	4,324
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 | Trading account debt securities | FHLMC
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	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 2
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	165,282	153,966	546,335	307,512
Level 2 | Trading account debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	165,282	153,966	546,335	307,512
Level 2 | Trading account debt securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	84,896	99,576	293,613	213,608
Level 2 | Trading account debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	76,118	54,390	234,953	93,904
Level 2 | Trading account debt securities | FHLMC
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	4,268		17,769
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 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,597	1,737	8,286	4,324
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 | Trading account debt securities | FHLMC
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0		0
Level 3 | Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 2,597	$ 1,737	$ 8,286	$ 4,324