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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities (gains) losses reclassified into earnings									$ (4,320)	$ 900	$ (567)
Provision for income taxes	$ 7,046	$ 8,334	$ 10,103	$ 8,858	$ 10,556	$ 6,516	$ 7,335	$ 7,004	34,341	31,412	53,807
Interest and dividends on securities (Interest and dividend income)									(85,654)	(77,676)	(58,120)
Employee benefits (Non-interest expense)									39,313	30,079	29,933
Net effect on accumulated other comprehensive income/(loss) for the period									1,970	1,949	1,958
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									(175)	11	(27)
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									(168)	(188)	(209)
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									2,313	2,126	2,194
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									(227)	15	(42)
Provision for income taxes									52	(4)	15
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]
Provision for income taxes									54	56	117
Interest and dividends on securities (Interest and dividend income)									(222)	(244)	(326)
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]
Provision for income taxes									(729)	(822)	(1,053)
Employee benefits (Non-interest expense)									$ 3,042	$ 2,948	$ 3,247