Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale, Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investment Securities [Line Items]
Amortized Cost	$ 896,495	$ 924,054
Securities available for sale	902,292	$ 909,247
US Government Agencies Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	194,100
Securities available for sale	197,500
Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	660,800
Securities available for sale	$ 661,900