Note Transfers of financial assets and servicing rights (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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 108,091	$ 213,348	$ 461,273	$ 601,329
GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	136,000	170,000	283,000	304,000
FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	21,000	43,000	86,000	79,000
Trading account securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	102,174	184,829	438,458	568,270
Trading account securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	85,722	140,255	368,660	444,382
Trading account securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	16,452	44,574	69,798	123,888
Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,588	2,695	6,766	7,235
Available For Sale Securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	4,329	25,824	16,049	25,824
Available For Sale Securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		20,686		20,686
Available For Sale Securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	4,329	5,138	16,049	5,138
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 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 Securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		0		0
Level 1 | Available For Sale 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	106,503	210,653	454,507	594,094
Level 2 | Trading account securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	102,174	184,829	438,458	568,270
Level 2 | Trading account securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	85,722	140,255	368,660	444,382
Level 2 | Trading account securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	16,452	44,574	69,798	123,888
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 Securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	4,329	25,824	16,049	25,824
Level 2 | Available For Sale Securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		20,686		20,686
Level 2 | Available For Sale Securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	4,329	5,138	16,049	5,138
Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	1,588	2,695	6,766	7,235
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	1,588	2,695	6,766	7,235
Level 3 | Available For Sale 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 Securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized		0		0
Level 3 | Available For Sale Securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 0	$ 0	$ 0	$ 0