Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Unrealized Losses On Securities [Line Items]
Estimated Fair Value, Less Than 12 Months	$ 166,470	$ 278,207
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(637)	(3,121)
Estimated Fair Value, 12 Months or More	235,380	733,222
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses	(2,950)	(24,933)
Total Estimated Fair Value	401,850	1,011,429
Total Gross Unrealized Losses	(3,587)	(28,054)
Corporate Securities [Member]
Schedule Of Unrealized Losses On Securities [Line Items]
Estimated Fair Value, Less Than 12 Months		14,769
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses		(214)
Estimated Fair Value, 12 Months or More		3,993
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses		(7)
Total Estimated Fair Value		18,762
Total Gross Unrealized Losses		(221)
Obligations of U.S. Government [Member] | Mortgage - Backed Securities - Residential [Member]
Schedule Of Unrealized Losses On Securities [Line Items]
Estimated Fair Value, Less Than 12 Months		2,289
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses		(41)
Estimated Fair Value, 12 Months or More		5,028
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses		(140)
Total Estimated Fair Value		7,317
Total Gross Unrealized Losses		(181)
Obligations of U.S. Government [Member] | Mortgage - Backed Securities - Commercial [Member]
Schedule Of Unrealized Losses On Securities [Line Items]
Estimated Fair Value, Less Than 12 Months	54,501	95,826
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(201)	(925)
Estimated Fair Value, 12 Months or More	16,365	75,959
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses	(149)	(3,505)
Total Estimated Fair Value	70,866	171,785
Total Gross Unrealized Losses	(350)	(4,430)
Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage - Backed Securities - Residential [Member]
Schedule Of Unrealized Losses On Securities [Line Items]
Estimated Fair Value, Less Than 12 Months	111,969	156,732
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(436)	(1,856)
Estimated Fair Value, 12 Months or More	219,015	626,003
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses	(2,801)	(20,789)
Total Estimated Fair Value	330,984	782,735
Total Gross Unrealized Losses	$ (3,237)	(22,645)
Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage - Backed Securities - Commercial [Member]
Schedule Of Unrealized Losses On Securities [Line Items]
Estimated Fair Value, Less Than 12 Months		0
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses		0
Estimated Fair Value, 12 Months or More		12,901
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses		(258)
Total Estimated Fair Value		12,901
Total Gross Unrealized Losses		(258)
US States and Political Subdivisions Debt Securities [Member] | Other Government - Sponsored Enterprises [Member]
Schedule Of Unrealized Losses On Securities [Line Items]
Estimated Fair Value, Less Than 12 Months		8,591
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses		(85)
Estimated Fair Value, 12 Months or More		9,338
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses		(234)
Total Estimated Fair Value		17,929
Total Gross Unrealized Losses		$ (319)