Summary of Carrying Values, Estimated Fair Values and Fair Value Measurement Levels of Financial Instruments (Detail) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016		Sep. 30, 2016	Dec. 31, 2015
Financial Assets:
Cash and cash equivalents		$ 3,277,427	$ 557,850		$ 771,782	$ 537,674
Securities held to maturity	[1]		3,712,776
FHLB stock	[2]	579,474	590,934
Loans, net		37,452,219	39,308,016
Financial Liabilities:
Deposits		28,893,197	28,887,903
Borrowed funds		12,363,602	13,673,379
Financial Assets:
Cash and cash equivalents		3,277,427	557,850
Securities held to maturity	[1]		3,813,959
FHLB stock	[2]	579,474	590,934
Loans, net		37,671,152	39,416,469
Financial Liabilities:
Deposits		28,869,413	28,888,064
Borrowed funds		12,277,697	13,633,943
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Assets:
Cash and cash equivalents		3,277,427	557,850
Securities held to maturity			200,220
Financial Liabilities:
Deposits	[3]	20,090,624	21,310,733
Significant Other Observable Inputs (Level 2)
Financial Assets:
Securities held to maturity			3,613,739
FHLB stock	[2]	579,474	590,934
Financial Liabilities:
Deposits		8,778,789	7,577,331	[4]
Borrowed funds		12,277,697	13,633,943
Significant Unobservable Inputs (Level 3)
Financial Assets:
Loans, net		$ 37,671,152	$ 39,416,469

[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 December 31, 2016, the non-credit portion of OTTI recorded in AOCL was $8.6 million (before taxes).

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

[3] Interest-bearing checking and money market accounts, savings accounts, and non-interest-bearing accounts.

[4] Certificates of deposit.