Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Investment Securities [Line Items]
Amortized Cost	$ 805,515	$ 896,495
Gross Unrealized Gains	25,708	8,906
Gross Unrealized Losses	0	(3,109)
Securities available for sale	831,223	902,292
Estimated Fair Value, Less Than 12 Months	3,755	166,470
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(10)	(637)
Estimated Fair Value, 12 Months or More	0	235,380
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses	0	(2,950)
Securities Continuous Unrealized Loss Position Fair Value	3,755	401,850
SecuritiesContinuousUnrealizedLossPositionAggregateLosses	(10)	(3,587)
US States and Political Subdivisions Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	11,154	17,738
Gross Unrealized Gains	243	171
Gross Unrealized Losses	0	0
Securities available for sale	11,397	17,909
Corporate Securities [Member]
Investment Securities [Line Items]
Amortized Cost	22,941	22,919
Gross Unrealized Gains	1,444	1,043
Gross Unrealized Losses	0	0
Securities available for sale	24,385	23,962
Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage - Backed Securities - Residential [Member]
Investment Securities [Line Items]
Amortized Cost	481,109	660,777
Gross Unrealized Gains	14,924	4,113
Gross Unrealized Losses	0	(2,943)
Securities available for sale	496,033	661,947
Estimated Fair Value, Less Than 12 Months	3,755	111,969
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(10)	(436)
Estimated Fair Value, 12 Months or More	0	219,015
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses	0	(2,801)
Securities Continuous Unrealized Loss Position Fair Value	3,755	330,984
SecuritiesContinuousUnrealizedLossPositionAggregateLosses	(10)	(3,237)
Obligations of U.S. Government-Sponsored Enterprises [Member] | Other Government - Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	100,996	1,000
Gross Unrealized Gains	2	0
Gross Unrealized Losses	0	0
Securities available for sale	100,998	1,000
Obligations of U.S. Government [Member] | Mortgage - Backed Securities - Commercial [Member]
Investment Securities [Line Items]
Amortized Cost	182,823	186,316
Gross Unrealized Gains	8,357	2,983
Gross Unrealized Losses	0	(166)
Securities available for sale	191,180	189,133
Estimated Fair Value, Less Than 12 Months		54,501
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses		(201)
Estimated Fair Value, 12 Months or More		16,365
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses		(149)
Securities Continuous Unrealized Loss Position Fair Value		70,866
SecuritiesContinuousUnrealizedLossPositionAggregateLosses		(350)
Obligations of U.S. Government [Member] | Mortgage - Backed Securities - Residential [Member]
Investment Securities [Line Items]
Amortized Cost	6,492	7,745
Gross Unrealized Gains	738	596
Gross Unrealized Losses	0	0
Securities available for sale	$ 7,230	$ 8,341