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, 2018	Mar. 31, 2017
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	$ 140,657	$ 177,090
GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	112,000	147,000
FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	26,000	28,000
Trading account securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	132,520	169,868
Trading account securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	112,495	146,977
Trading account securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	20,025	22,891
Mortgage Servicing Rights
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,415	2,470
Available-for-sale debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	5,722	4,752
Available-for-sale debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	5,722	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 debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	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
Level 2
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	138,242	174,620
Level 2 | Trading account securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	132,520	169,868
Level 2 | Trading account securities | GNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	112,495	146,977
Level 2 | Trading account securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	20,025	22,891
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 debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	5,722	4,752
Level 2 | Available-for-sale debt securities | FNMA
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	5,722	4,752
Level 3
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	2,415	2,470
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	2,415	2,470
Level 3 | Available-for-sale debt securities
Assets
Initial fair value of the assets obtained as proceeds from residential mortgage loans securitized	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