Summary of Carrying Values, Estimated Fair Values and Fair Value Measurement Levels of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014	Dec. 31, 2013
Financial Assets:
Cash and cash equivalents	$ 582,558		$ 564,150		$ 672,872	$ 644,550
Securities held to maturity	6,784,883	[1]	6,922,667	[2]
FHLB stock	464,942	[3]	515,327	[3]
Loans, net	35,519,143		35,647,639
Cash and cash equivalents	582,558		564,150
Securities held to maturity	7,028,201	[1]	7,085,971	[2]
FHLB stock	464,942	[3]	515,327	[3]
Loans, net	36,050,313		36,167,980
Financial Liabilities:
Deposits	28,931,387		28,328,734
Borrowed funds	13,264,402		14,226,487
Deposits	28,984,723		28,377,897
Borrowed funds	14,251,356		15,140,171
Financial Assets:
Cash and cash equivalents	582,558		564,150		672,872	644,550
Securities held to maturity	6,784,883	[1]	6,922,667	[2]
FHLB stock	464,942	[3]	515,327	[3]
Loans, net	35,519,143		35,647,639
Cash and cash equivalents	582,558		564,150
Securities held to maturity	7,028,201	[1]	7,085,971	[2]
FHLB stock	464,942	[3]	515,327	[3]
Loans, net	36,050,313		36,167,980
Financial Liabilities:
Deposits	28,931,387		28,328,734
Borrowed funds	13,264,402		14,226,487
Deposits	28,984,723		28,377,897
Borrowed funds	14,251,356		15,140,171
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Assets:
Cash and cash equivalents	582,558		564,150
Financial Liabilities:
Deposits	23,082,998	[4]	21,908,136	[4]
Financial Assets:
Cash and cash equivalents	582,558		564,150
Financial Liabilities:
Deposits	23,082,998	[4]	21,908,136	[4]
Significant Other Observable Inputs (Level 2)
Financial Assets:
Securities held to maturity	7,027,192		7,084,959
FHLB stock	464,942	[3]	515,327	[3]
Financial Liabilities:
Deposits	5,901,725	[5]	6,469,761	[5]
Borrowed funds	14,251,356		15,140,171
Financial Assets:
Securities held to maturity	7,027,192		7,084,959
FHLB stock	464,942	[3]	515,327	[3]
Financial Liabilities:
Deposits	5,901,725	[5]	6,469,761	[5]
Borrowed funds	14,251,356		15,140,171
Significant Unobservable Inputs (Level 3)
Financial Assets:
Securities held to maturity	1,009		1,012
Loans, net	36,050,313		36,167,980
Financial Assets:
Securities held to maturity	1,009		1,012
Loans, net	$ 36,050,313		$ 36,167,980

[1] Held-to-maturity securities are reported at a carrying amount equal to amortized cost less the non-credit portion of OTTI recorded in AOCL. At March 31, 2015, the non-credit portion of OTTI recorded in AOCL was $8.8 million (before tax).

[2] Held-to-maturity securities are reported at a carrying amount equal to amortized cost less the non-credit portion of OTTI recorded in AOCL. At December 31, 2014, the non-credit portion of OTTI recorded in AOCL was $8.8 million (before taxes).

[3] Carrying value and estimated fair value are at cost.

[4] NOW and money market accounts, savings accounts, and non-interest-bearing accounts.

[5] Certificates of deposit.