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
	Mar. 31, 2020	Mar. 31, 2019
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 85,708	$ 93,725
GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	51,000	34,000
FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	71,000	21,000
Trading account debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	84,221	92,067
Trading account debt securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	50,648	71,149
Trading account debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	33,573	20,918
Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,487	1,658
Level 1
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	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
Level 1 | Trading account debt securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	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
Level 1 | Mortgage Servicing Rights
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	84,221	92,067
Level 2 | Trading account debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	84,221	92,067
Level 2 | Trading account debt securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	50,648	71,149
Level 2 | Trading account debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	33,573	20,918
Level 2 | Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0
Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,487	1,658
Level 3 | Trading account debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	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
Level 3 | Trading account debt securities | FNMA
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	$ 1,487	$ 1,658