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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 138,659	$ 108,091	$ 399,278	$ 461,273
GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	110,000	86,000	320,000	369,000
FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	26,000	21,000	72,000	86,000
Trading account securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	133,536	102,174	382,622	438,458
Trading account securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	109,911	85,722	319,769	368,660
Trading account securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	23,625	16,452	62,853	69,798
Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,625	1,588	7,198	6,766
Available-for-sale debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,498	4,329	9,458	16,049
Available-for-sale debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,498	4,329	9,458	16,049
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	136,034	106,503	392,080	454,507
Level 2 | Trading account securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	133,536	102,174	382,622	438,458
Level 2 | Trading account securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	109,911	85,722	319,769	368,660
Level 2 | Trading account securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	23,625	16,452	62,853	69,798
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	2,498	4,329	9,458	16,049
Level 2 | Available-for-sale debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,498	4,329	9,458	16,049
Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,625	1,588	7,198	6,766
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	2,625	1,588	7,198	6,766
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