Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive (Loss) Income, Net of Taxes (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive (Loss) Income [Line Items]
Beginning balance		$ (21,730)
Cumulative effect on retained earnings		692,472	$ 675,122	$ 624,133	$ 603,374
Other comprehensive income (loss)		(651)	6,603	(6,291)
Ending balance		(22,144)	(21,730)
Net Unrealized Holding (Losses) Gains on Available-for-Sale Investment Securities [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Beginning balance		(3,231)	(11,221)	(4,061)
Other comprehensive income (loss)		1,615	7,990	(5,759)
Ending balance		(1,379)	(3,231)	(11,221)
Net Change Related to Derivative Used for Cash Flow Hedge [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Beginning balance		(185)	81	9
Other comprehensive income (loss)		(236)	(349)	70
Ending balance		(421)	(185)	81
Net Change Related to Defined Benefit Pension Plan [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Beginning balance		(18,314)	(17,276)	(13,719)
Other comprehensive income (loss)		(2,030)	(1,038)	(602)
Ending balance		(20,344)	(18,314)	(17,276)
Accumulated Other Comprehensive (Loss) Income [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Beginning balance		(21,730)	(28,416)	(17,771)
Cumulative effect on retained earnings		(22,144)	(21,730)	(28,416)	(17,771)
Other comprehensive income (loss)		(651)	6,603	(6,291)
Ending balance		$ (22,144)	(21,730)	(28,416)
Accounting Standards Update 2016-01 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings					0
Accounting Standards Update 2016-01 [Member] | Net Unrealized Holding (Losses) Gains on Available-for-Sale Investment Securities [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings					(433)
Accounting Standards Update 2016-01 [Member] | Net Change Related to Derivative Used for Cash Flow Hedge [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings					0
Accounting Standards Update 2016-01 [Member] | Net Change Related to Defined Benefit Pension Plan [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings					0
Accounting Standards Update 2016-01 [Member] | Accumulated Other Comprehensive (Loss) Income [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings					(433)
Accounting Standards Update 2018-02 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings					0
Accounting Standards Update 2018-02 [Member] | Net Unrealized Holding (Losses) Gains on Available-for-Sale Investment Securities [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings					(968)
Accounting Standards Update 2018-02 [Member] | Net Change Related to Derivative Used for Cash Flow Hedge [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings					2
Accounting Standards Update 2018-02 [Member] | Net Change Related to Defined Benefit Pension Plan [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings					(2,955)
Accounting Standards Update 2018-02 [Member] | Accumulated Other Comprehensive (Loss) Income [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings					$ (3,921)
Accounting Standards Update 2017-12 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings				0
Accounting Standards Update 2017-12 [Member] | Net Unrealized Holding (Losses) Gains on Available-for-Sale Investment Securities [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings				0
Accounting Standards Update 2017-12 [Member] | Net Change Related to Derivative Used for Cash Flow Hedge [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings				83
Accounting Standards Update 2017-12 [Member] | Net Change Related to Defined Benefit Pension Plan [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings				0
Accounting Standards Update 2017-12 [Member] | Accumulated Other Comprehensive (Loss) Income [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings				$ 83
Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings			(11,047)
Accounting Standards Update 2016-13 | Net Unrealized Holding (Losses) Gains on Available-for-Sale Investment Securities [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings	[1]		237
Accounting Standards Update 2016-13 | Net Change Related to Derivative Used for Cash Flow Hedge [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings	[1]		0
Accounting Standards Update 2016-13 | Net Change Related to Defined Benefit Pension Plan [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings	[1]		0
Accounting Standards Update 2016-13 | Accumulated Other Comprehensive (Loss) Income [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive (Loss) Income [Line Items]
Cumulative effect on retained earnings	[1]		$ 237

[1] (1) See Note 1, "Summary of Significant Accounting Policies - Accounting Pronouncements Adopted in 2020" for additional information.