Summary of Carrying Values, Estimated Fair Values and Fair Value Measurement Levels of Financial Instruments (Detail) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Financial Assets:
Cash and cash equivalents		$ 2,204,397	$ 2,528,169
FHLB stock	[1]	653,075	603,819
Loans, net		39,287,173	38,265,183
Financial Liabilities:
Deposits		29,555,845	29,102,163
Borrowed funds		13,793,839	12,913,679
Financial Assets:
Cash and cash equivalents		2,204,397	2,528,169
FHLB stock	[1]	653,075	603,819
Loans, net		38,834,022	38,254,538
Financial Liabilities:
Deposits		29,503,131	29,044,852
Borrowed funds		13,649,155	12,780,653
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Assets:
Cash and cash equivalents		2,204,397	2,528,169
Financial Liabilities:
Deposits	[2]	19,249,326	20,458,517
Significant Other Observable Inputs (Level 2)
Financial Assets:
FHLB stock	[1]	653,075	603,819
Financial Liabilities:
Deposits	[3]	10,253,805	8,586,335
Borrowed funds		13,649,155	12,780,653
Significant Unobservable Inputs (Level 3)
Financial Assets:
Loans, net		$ 38,834,022	$ 38,254,538

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

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

[3] Certificates of deposit.