Summary of Carrying Values, Estimated Fair Values and Fair Value Measurement Levels of Financial Instruments (Detail) - USD ($) $ in Thousands		Sep. 30, 2016		Dec. 31, 2015		Sep. 30, 2015	Dec. 31, 2014
Financial Assets:
Cash and cash equivalents		$ 771,782		$ 537,674		$ 585,794	$ 564,150
Securities held to maturity		3,651,925	[1]	5,969,390	[2]
FHLB stock	[3]	586,355		663,971
Loans, net		39,671,237		38,011,995
Financial Liabilities:
Deposits		29,139,599		28,426,758
Borrowed funds		14,001,809		15,748,405
Financial Assets:
Cash and cash equivalents		771,782		537,674
Securities held to maturity		3,893,086	[1]	6,108,529	[2]
FHLB stock	[3]	586,355		663,971
Loans, net		40,099,680		38,245,434
Financial Liabilities:
Deposits		29,156,152		28,408,915
Borrowed funds		14,057,006		15,685,616
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Assets:
Cash and cash equivalents		771,782		537,674
Financial Liabilities:
Deposits	[4]	21,755,869		23,114,271
Significant Other Observable Inputs (Level 2)
Financial Assets:
Securities held to maturity		3,892,670		6,107,697
FHLB stock	[3]	586,355		663,971
Financial Liabilities:
Deposits	[5]	7,400,283		5,294,644
Borrowed funds		14,057,006		15,685,616
Significant Unobservable Inputs (Level 3)
Financial Assets:
Securities held to maturity		416		832
Loans, net		$ 40,099,680		$ 38,245,434

[1] Held-to-maturity securities are reported at a carrying amount equal to amortized cost less the non-credit portion of OTTI recorded in Accumulated Other Comprehensive Loss ("AOCL"). At September 30, 2016, the non-credit portion of OTTI recorded in AOCL was $8.6 million, pre-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, 2015, the non-credit portion of OTTI recorded in AOCL was $8.7 million, pre-tax.

[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.