Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Unrealized Losses On Securities [Line Items]
Estimated Fair Value, Less Than 12 Months	$ 278,207	$ 431,247
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(3,121)	(3,386)
Estimated Fair Value, 12 Months or More	733,222	456,539
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses	(24,933)	(15,148)
Total Estimated Fair Value	1,011,429	887,786
Total Gross Unrealized Losses	(28,054)	(18,534)
Corporate Securities [Member]
Schedule Of Unrealized Losses On Securities [Line Items]
Estimated Fair Value, Less Than 12 Months	14,769	3,993
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(214)	(4)
Estimated Fair Value, 12 Months or More	3,993	0
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses	(7)	0
Total Estimated Fair Value	18,762	3,993
Total Gross Unrealized Losses	(221)	(4)
Pooled Trust Preferred Collateralized Debt Obligations [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		19,120
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses		(4,379)
Total Estimated Fair Value		19,120
Total Gross Unrealized Losses		(4,379)
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	5,584
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(41)	(21)
Estimated Fair Value, 12 Months or More	5,028	0
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses	(140)	0
Total Estimated Fair Value	7,317	5,584
Total Gross Unrealized Losses	(181)	(21)
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	95,826	48,322
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(925)	(962)
Estimated Fair Value, 12 Months or More	75,959	32,683
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses	(3,505)	(894)
Total Estimated Fair Value	171,785	81,005
Total Gross Unrealized Losses	(4,430)	(1,856)
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	156,732	351,222
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(1,856)	(2,295)
Estimated Fair Value, 12 Months or More	626,003	400,984
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses	(20,789)	(9,746)
Total Estimated Fair Value	782,735	752,206
Total Gross Unrealized Losses	(22,645)	(12,041)
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	13,985
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	0	(71)
Estimated Fair Value, 12 Months or More	12,901	0
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses	(258)	0
Total Estimated Fair Value	12,901	13,985
Total Gross Unrealized Losses	(258)	(71)
Obligations of U.S. Government-Sponsored Enterprises [Member] \| Other Government - Sponsored Enterprises [Member]
Schedule Of Unrealized Losses On Securities [Line Items]
Estimated Fair Value, Less Than 12 Months		997
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses		(1)
Estimated Fair Value, 12 Months or More		99
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses		0
Total Estimated Fair Value		1,096
Total Gross Unrealized Losses		(1)
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	7,144
Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses	(85)	(32)
Estimated Fair Value, 12 Months or More	9,338	3,653
SecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses	(234)	(129)
Total Estimated Fair Value	17,929	10,797
Total Gross Unrealized Losses	$ (319)	$ (161)

Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($)
$ in Thousands

Dec. 31, 2018

Dec. 31, 2017

Schedule Of Unrealized Losses On Securities [Line Items]

Estimated Fair Value, Less Than 12 Months

$ 278,207

$ 431,247

Available For Sale Securities Continuous Unrealized Income Loss Position Less Than Twelve Months Aggregate Gain Losses

(3,121)

(3,386)

Estimated Fair Value, 12 Months or More

733,222