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
	Mar. 31, 2017	Mar. 31, 2016
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 177,090	$ 172,118
GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	147,000	134,000
FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	28,000	36,000
Trading account securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	169,868	170,248
Trading account securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	146,977	134,012
Trading account securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	22,891	36,236
Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,470	1,870
Available For Sale Securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	4,752
Available For Sale Securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	4,752
Level 1
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0
Level 1 | Trading account securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	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
Level 1 | Trading account 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 | Available For Sale Securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0
Level 1 | Available For Sale Securities | 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	174,620	170,248
Level 2 | Trading account securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	169,868	170,248
Level 2 | Trading account securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	146,977	134,012
Level 2 | Trading account securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	22,891	36,236
Level 2 | Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0	0
Level 2 | Available For Sale Securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	4,752
Level 2 | Available For Sale Securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	4,752
Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,470	1,870
Level 3 | Trading account securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	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
Level 3 | Trading account 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	2,470	$ 1,870
Level 3 | Available For Sale Securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	0
Level 3 | Available For Sale Securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 0