Comprehensive Income/(Loss) - Schedule of Amounts Reclassified from Accumulated Other Comprehensive Income/(Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities (gains) losses reclassified into earnings									$ (4,268)	$ (4,320)	$ 900
Employee benefits (Non-interest expense)									41,723	39,313	30,079
Interest and dividends on securities (Interest and dividend income)									(70,593)	(85,654)	(77,676)
Provision for income taxes	$ 11,703	$ 7,669	$ 42	$ 3,621	$ 7,046	$ 8,334	$ 10,103	$ 8,858	23,035	34,341	31,412
Net effect on accumulated other comprehensive income/(loss) for the period									325	1,970	1,949
Accumulated Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net effect on accumulated other comprehensive income/(loss) for the period									(1,936)	(175)	11
Accumulated Defined Benefit Plans Adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net effect on accumulated other comprehensive income/(loss) for the period									2,286	2,313	2,126
Accumulated Unrealized Gains on Debt Securities Transferred from Available For Sale to Held to Maturity [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net effect on accumulated other comprehensive income/(loss) for the period									(25)	(168)	(188)
Amounts Reclassified From Accumulated Other Comprehensive Income/(Loss) [Member] | Accumulated Unrealized Gains (Losses) on Debt Securities Available for Sale [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities (gains) losses reclassified into earnings									(2,540)	(227)	15
Provision for income taxes									604	52	(4)
Amounts Reclassified From Accumulated Other Comprehensive Income/(Loss) [Member] | Accumulated Defined Benefit Plans Adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Employee benefits (Non-interest expense)									3,000	3,042	2,948
Provision for income taxes									(714)	(729)	(822)
Amounts Reclassified From Accumulated Other Comprehensive Income/(Loss) [Member] | Accumulated Unrealized Gains on Debt Securities Transferred from Available For Sale to Held to Maturity [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest and dividends on securities (Interest and dividend income)									(32)	(222)	(244)
Provision for income taxes									$ 7	$ 54	$ 56