Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investment Securities [Line Items]
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	$ 104,991
Amortized Cost	880,416	$ 896,495
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	27,832	8,906
Gross Unrealized Losses	(12)	(3,109)
Securities available for sale, at fair value	908,236	902,292
Due within one year, Estimated Fair Value	105,041
Due after one but within five years, Amortized Cost	20,225
Due after one but within five years, Estimated Fair Value	21,260
Due after five but within ten years, Amortized Cost	7,127
Due after five but within ten years, Estimated Fair Value	7,749
Due after ten years, Amortized Cost	4,550
Due after ten years, Estimated Fair Value	4,550
Due within one year, Amortized Cost	136,893
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value	138,600
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	743,523
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	769,636
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	216,000
Securities available for sale, at fair value	226,100
Residential Mortgage Backed Securities [Member]
Investment Securities [Line Items]
Securities available for sale, at fair value	543,543	661,947
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Securities available for sale, at fair value	100,993	1,000
Obligations of States and Political Subdivisions [Member]
Investment Securities [Line Items]
Securities available for sale, at fair value	13,167	17,909
Corporate Debt Securities [Member]
Investment Securities [Line Items]
Securities available for sale, at fair value	24,440	23,962
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	880,416
Securities available for sale, at fair value	908,236
US Government Agency Debt Securities Commercial [Domain]
Investment Securities [Line Items]
Amortized Cost	209,175	186,316
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	9,377	2,983
Gross Unrealized Losses	0	(166)
Securities available for sale, at fair value	218,552	189,133
Corporate Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	22,935	22,919
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,505	1,043
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	24,440	23,962
Obligations of States and Political Subdivisions [Member]
Investment Securities [Line Items]
Amortized Cost	12,965	17,738
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	208	171
Gross Unrealized Losses	(6)	0
Securities available for sale, at fair value	13,167	17,909
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	100,993	1,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	2	0
Gross Unrealized Losses	(2)	0
Securities available for sale, at fair value	100,993	1,000
Residential Mortgage Backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	527,570	660,777
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	15,977	4,113
Gross Unrealized Losses	(4)	(2,943)
Securities available for sale, at fair value	543,543	661,947
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	6,778	7,745
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	763	596
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	$ 7,541	$ 8,341

Investment Securities [Line Items]

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), without single maturity date and not allocated over maturity grouping.

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