Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale, Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investment Securities [Line Items]
Amortized Cost	$ 805,515	$ 896,495
Securities available for sale	831,223	$ 902,292
US Government Agencies Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	189,300
Securities available for sale	198,400
Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	481,100
Securities available for sale	$ 496,000