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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 119,962	$ 176,092	$ 260,619	$ 353,182
GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	97,000	136,000	210,000	283,000
FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	20,000	37,000	46,000	65,000
Trading account securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	116,566	166,416	249,086	336,284
Trading account securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	97,363	135,961	209,858	282,938
Trading account securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	19,203	30,455	39,228	53,346
Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,158	2,708	4,573	5,178
Available-for-sale debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,238	6,968	6,960	11,720
Available-for-sale debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,238	6,968	6,960	11,720
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 securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 1 | Trading account 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 securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	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 1 | Available-for-sale debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 1 | Available-for-sale debt securities | FNMA
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	117,804	173,384	256,046	348,004
Level 2 | Trading account securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	116,566	166,416	249,086	336,284
Level 2 | Trading account securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	97,363	135,961	209,858	282,938
Level 2 | Trading account securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	19,203	30,455	39,228	53,346
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 2 | Available-for-sale debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,238	6,968	6,960	11,720
Level 2 | Available-for-sale debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,238	6,968	6,960	11,720
Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,158	2,708	4,573	5,178
Level 3 | Trading account securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 3 | Trading account 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 securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 3 | Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,158	2,708	4,573	5,178
Level 3 | Available-for-sale debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0	0	0
Level 3 | Available-for-sale debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 0	$ 0	$ 0	$ 0