Investment Securities Held-to-Maturity - Unrealized Market Loss of Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	$ 38,086	$ 636,077
Less Than 12 Months, Unrealized Losses	94	6,798
12 Months or Longer, Fair Value	70,461	123,864
12 Months or Longer, Unrealized Losses	536	2,926
Total, Fair Value	108,547	759,941
Total, Unrealized Losses	630	9,724
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	14,999	9,859
Less Than 12 Months, Unrealized Losses	1	141
Total, Fair Value	14,999	9,859
Total, Unrealized Losses	1	141
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	23,087	626,218
Less Than 12 Months, Unrealized Losses	93	6,657
12 Months or Longer, Fair Value	70,461	123,864
12 Months or Longer, Unrealized Losses	536	2,926
Total, Fair Value	93,548	750,082
Total, Unrealized Losses	$ 629	$ 9,583