Investment Securities Held-to-Maturity - Unrealized Market Loss of Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	$ 204,764	$ 636,077
Less Than 12 Months, Unrealized Losses	661	6,798
12 Months or Longer, Fair Value	57,854	123,864
12 Months or Longer, Unrealized Losses	748	2,926
Total, Fair Value	262,618	759,941
Total, Unrealized Losses	1,409	9,724
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	41,984	9,859
Less Than 12 Months, Unrealized Losses	16	141
Total, Fair Value	41,984	9,859
Total, Unrealized Losses	16	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	162,780	626,218
Less Than 12 Months, Unrealized Losses	645	6,657
12 Months or Longer, Fair Value	57,854	123,864
12 Months or Longer, Unrealized Losses	748	2,926
Total, Fair Value	220,634	750,082
Total, Unrealized Losses	$ 1,393	$ 9,583