Other Comprehensive Income (Components of AOCI) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,619,797	$ 1,413,840
Current - period other comprehensive (loss) income	(4,678)	11,117
Ending Balance	1,647,231	1,412,589
Unrealized Gains (Losses) on Available for Sale Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	23,690	8,746
Current - period other comprehensive (loss) income	(9,190)	16,746
Ending Balance	14,500	25,492
Post- Retirement Obligations
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,081)	(5,240)
Current - period other comprehensive (loss) income	(109)	84
Ending Balance	(1,190)	(5,156)
Unrealized (Losses) Gains on Derivatives (cash flow hedges)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(4,954)	315
Current - period other comprehensive (loss) income	4,621	(5,713)
Ending Balance	(333)	(5,398)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	17,655	3,821
Current - period other comprehensive (loss) income	(4,678)	11,117
Ending Balance	$ 12,977	$ 14,938