Notes payable (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011		Mar. 31, 2011
Debt Instrument [Line Items]
Others	$ 24,896		$ 25,070		$ 25,206
Notes Payable	1,843,754		1,856,372		3,794,655
Fixed Rate 1.07% To 4.95% Advances Due 2012 To 2021 [Member]
Debt Instrument [Line Items]
Federal Home Loan Bank Advances Long Term	623,286		642,568		577,000
Paying Interest Monthly At An Annual Fixed Rate 2.50% | Federal Deposit Insurance Corporation [Member]
Debt Instrument [Line Items]
Notes Payable	0		0		2,022,669
Fixed Rate 5.25% to 7.86% Term Notes Due 2012 to 2016
Debt Instrument [Line Items]
Notes Payable	278,337		278,309		278,201
Floating Rate 3.00% Term Notes Due 2012 to 2013
Debt Instrument [Line Items]
Notes Payable	478		588		907
Fixed Rate 6.125% to 8.327% Junior Subordinated Deferrable Interest Debentures Due 2027 to 2034
Debt Instrument [Line Items]
Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust	439,800		439,800		439,800
Fixed Rate 5.00% Until December 2013 And 9.00% Thereafter Junior Subordinated Deferrable Interest Debentures with no Stated Maturity
Debt Instrument [Line Items]
Junior Subordinated Debenture Owed To Unconsolidated Subsidiary Trust	$ 476,957	[1]	$ 470,037	[1]	$ 450,872	[1]

[1] The debentures are perpetual and may be redeemed by the Corporation at any time, subject to the consent of the Board of Governors of the Federal Reserve System. The discount on the debentures is being amortized over an estimated 30-year term that started in August 2009. The effective interest rate, including the discount accretion, was approximately 16% at March 31, 2012 and 2011.