Other Comprehensive Income (Components of AOCI) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,677,634	$ 1,410,407	$ 1,619,797	$ 1,413,840
Current - period other comprehensive (loss) income	(6,325)	537	(9,898)	9,510
Ending Balance	1,679,424	1,601,569	1,679,424	1,601,569
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	14,082	12,794	17,655	3,821
Current - period other comprehensive (loss) income	(6,325)	537	(9,898)	9,510
Ending Balance	7,757	13,331	7,757	13,331
Unrealized Gains (Losses) on Available for Sale Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	16,940	24,562	23,690	8,746
Current - period other comprehensive (loss) income	(7,990)	(428)	(14,740)	15,388
Ending Balance	8,950	24,134	8,950	24,134
Post- Retirement Obligations
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(1,301)	(5,086)	(1,081)	(5,240)
Current - period other comprehensive (loss) income	(103)	85	(323)	239
Ending Balance	(1,404)	(5,001)	(1,404)	(5,001)
Unrealized (Losses) Gains on Derivatives (cash flow hedges)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(1,557)	(6,682)	(4,954)	315
Current - period other comprehensive (loss) income	1,768	880	5,165	(6,117)
Ending Balance	$ 211	$ (5,802)	$ 211	$ (5,802)