Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale, Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investment Securities [Line Items]
Amortized Cost	$ 924,054	$ 740,846
Securities available for sale	909,247	$ 731,358
US Government Agencies Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	178,600
Securities available for sale	177,200
Obligations of U.S. Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	686,900
Securities available for sale	$ 673,400