Comprehensive Income/(Loss) - Schedule of Amounts Reclassified from Accumulated Other Comprehensive Income/(Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains reclassified into earnings	$ (235)	$ (84)	$ (3,800)	$ (403)
Provision for income taxes	8,334	6,516	27,295	20,855
Employee benefits (Non-interest expense)	9,726	7,905	29,419	22,782
Interest and dividends on securities (Interest and dividend income)	(20,646)	(20,290)	(64,504)	(55,918)
Net effect on accumulated other comprehensive income/(loss) for the period	355	537	1,343	1,463
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	(169)	(9)	(199)	(9)
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	(68)	(49)	(179)	(149)
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	592	595	1,721	1,621
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 reclassified into earnings	(219)	(11)	(258)	(11)
Provision for income taxes	50	2	59	2
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	20	15	58	46
Interest and dividends on securities (Interest and dividend income)	(88)	(64)	(237)	(195)
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	(175)	(177)	(554)	(672)
Employee benefits (Non-interest expense)	$ 767	$ 772	$ 2,275	$ 2,293