Note Transfers of financial assets and servicing assets (Initial fair value of assets obtained as proceeds from residential mortgage loans securitized) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 93,725	$ 140,657
GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	71,000	112,000
FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	21,000	26,000
Trading account debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	92,067	132,520
Trading account debt securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	71,149	112,495
Trading account debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	20,918	20,025
Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,658	2,415
Debt securities available-for-sale
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		5,722
Debt securities available-for-sale | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		5,722
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 1 | Debt securities available-for-sale
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		0
Level 1 | Debt securities available-for-sale | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		0
Level 2
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	92,067	138,242
Level 2 | Trading account debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	92,067	132,520
Level 2 | Trading account debt securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	71,149	112,495
Level 2 | Trading account debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	20,918	20,025
Level 2 | Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0
Level 2 | Debt securities available-for-sale
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		5,722
Level 2 | Debt securities available-for-sale | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		5,722
Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,658	2,415
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	$ 1,658	2,415
Level 3 | Debt securities available-for-sale
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		0
Level 3 | Debt securities available-for-sale | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		$ 0