Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale, Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investment Securities [Line Items]
Amortized Cost	$ 790,082	$ 891,111
Estimated Fair Value of Available for Sale	778,612	$ 886,560
US Government Agencies Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	15,100
Estimated Fair Value of Available for Sale	16,600
Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	683,600
Estimated Fair Value of Available for Sale	$ 676,900