Comprehensive Income/(Loss) - Schedule of Amounts Reclassified from Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains (Non-interest income)	$ (358)	$ (494)	$ (319)	$ (506)
Interest and dividends on securities (Interest and dividend income)	(19,249)	(14,239)	(35,628)	(28,726)
Employee benefits (Non-interest expense)	7,965	7,731	14,877	15,941
Provision for income taxes	7,335	9,653	14,339	20,274
Net effect on accumulated other comprehensive income for the period	538	470	926	1,076
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 for the period		35	0	35
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 for the period	(51)	(74)	(100)	(123)
Accumulated Defined Benefit Plans Adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net effect on accumulated other comprehensive income for the period	589	509	1,026	1,164
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 (Non-interest income)		55		55
Provision for income taxes		(20)		(20)
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)	(66)	(118)	(131)	(189)
Provision for income taxes	15	44	31	66
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)	764	809	1,520	1,610
Provision for income taxes	$ (175)	$ (300)	$ (494)	$ (446)