Initial fair value of assets obtained as proceeds from residential mortgage loans securitized (Detail) (Residential Mortgage, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	$ 205,976	$ 231,713
Trading Account Assets[Member]
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	203,414	228,515
Trading Account Assets[Member] | Government National Mortgage Association Certificates And Obligations GNMA Member
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	156,456	165,932
Trading Account Assets[Member] | Federal National Mortgage Association Certificates And Obligations FNMA Member
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	46,958	62,583
Servicing Asset At Fair Value Amount [Member]
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	2,562	3,198
Fair Value, Inputs, Level 1 [Member]
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	0	0
Fair Value, Inputs, Level 1 [Member] | Trading Account Assets[Member]
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	0	0
Fair Value, Inputs, Level 1 [Member] | Trading Account Assets[Member] | Government National Mortgage Association Certificates And Obligations GNMA Member
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	0	0
Fair Value, Inputs, Level 1 [Member] | Trading Account Assets[Member] | Federal National Mortgage Association Certificates And Obligations FNMA Member
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	0	0
Fair Value, Inputs, Level 1 [Member] | Servicing Asset At Fair Value Amount [Member]
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	0	0
Fair Value, Inputs, Level 2 [Member]
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	203,414	228,515
Fair Value, Inputs, Level 2 [Member] | Trading Account Assets[Member]
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	203,414	228,515
Fair Value, Inputs, Level 2 [Member] | Trading Account Assets[Member] | Government National Mortgage Association Certificates And Obligations GNMA Member
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	156,456	165,932
Fair Value, Inputs, Level 2 [Member] | Trading Account Assets[Member] | Federal National Mortgage Association Certificates And Obligations FNMA Member
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	46,958	62,583
Fair Value, Inputs, Level 2 [Member] | Servicing Asset At Fair Value Amount [Member]
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	0	0
Fair Value, Inputs, Level 3 [Member]
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	2,562	3,198
Fair Value, Inputs, Level 3 [Member] | Trading Account Assets[Member]
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	0	0
Fair Value, Inputs, Level 3 [Member] | Trading Account Assets[Member] | Government National Mortgage Association Certificates And Obligations GNMA Member
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	0	0
Fair Value, Inputs, Level 3 [Member] | Trading Account Assets[Member] | Federal National Mortgage Association Certificates And Obligations FNMA Member
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	0	0
Fair Value, Inputs, Level 3 [Member] | Servicing Asset At Fair Value Amount [Member]
Assets
Transfers Of Financial Assets Accounted For As Sale Initial Fair Value Of Asset Obtained As Proceeds	$ 2,562	$ 3,198