Change in Accumulated Other Comprehensive Income - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 1,850,306
Other comprehensive (loss) income before reclassifications	39,959	$ 39,563	$ (6,723)
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(7,453)	(668)	(519)
Net current-period other comprehensive loss	32,506	38,895	(7,242)
Ending Balance	1,791,726	1,850,306
Net change in investment securities available for sale
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	26,927	(14,553)	(7,842)
Other comprehensive (loss) income before reclassifications	39,853	41,733	(6,695)
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(6,898)	(253)	(16)
Net current-period other comprehensive loss	32,955	41,480	(6,711)
Ending Balance	59,882	26,927	(14,553)
Net change in investment securities held to maturity
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	468	779	1,223
Other comprehensive (loss) income before reclassifications	0	(2)	6
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(192)	(309)	(450)
Net current-period other comprehensive loss	(192)	(311)	(444)
Ending Balance	276	468	779
Net change in defined benefit plan
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(3,317)	834	865
Other comprehensive (loss) income before reclassifications	(1,445)	(4,045)	22
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(26)	(106)	(53)
Net current-period other comprehensive loss	(1,471)	(4,151)	(31)
Ending Balance	(4,788)	(3,317)	834
Net change in fair value of derivatives used for cash flow hedges (1)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(577)	(2,454)	(2,398)
Other comprehensive (loss) income before reclassifications	1,560	1,877	(56)
Less: Amounts reclassified from accumulated other comprehensive (loss) income	(337)	0	0
Net current-period other comprehensive loss	1,223	1,877	(56)
Ending Balance	646	(577)	(2,454)
Net change in equity method investments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	0	0	0
Other comprehensive (loss) income before reclassifications	(9)	0	0
Less: Amounts reclassified from accumulated other comprehensive (loss) income	0	0	0
Net current-period other comprehensive loss	(9)	0	0
Ending Balance	(9)	0	0
Accumulated Other Comprehensive (Loss) Income
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	23,501	(15,394)	(8,152)
Ending Balance	$ 56,007	$ 23,501	$ (15,394)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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