Other Comprehensive (Loss) Income (Components of AOCI) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,412,589	$ 1,373,816	$ 1,413,840	$ 1,358,980
Current - period other comprehensive income	(2,144)	9,060	8,973	16,312
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	1,412,589	1,391,446	1,413,840	1,358,980
Ending Balance	1,410,407	1,391,446	1,410,407	1,391,446
Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			(8,311)	4,350
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01			(8,311)	4,350
Unrealized Gains (Losses) on Available for Sale Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	25,492	(2,027)	8,746	(9,605)
Current - period other comprehensive income	(930)	9,662	15,816	17,240
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	24,562	7,635	24,562	(9,605)
Ending Balance	24,562	7,635	24,562	7,635
Post- Retirement Obligations
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(5,156)	(3,637)	(5,240)	(3,625)
Current - period other comprehensive income	70	34	154	22
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	(5,086)	(3,603)	(5,240)	(3,603)
Ending Balance	(5,086)	(3,603)	(5,086)	(3,603)
Unrealized Losses on Derivatives (cash flow hedges)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(5,398)	580	315	894
Current - period other comprehensive income	(1,284)	(636)	(6,997)	(950)
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	(6,682)	(56)	(6,682)	(56)
Ending Balance	(6,682)	(56)	(6,682)	(56)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	14,938	(5,084)	3,821	(12,336)
Current - period other comprehensive income	(2,144)	9,060	8,973	16,312
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	12,794	3,976	12,794	3,976
Ending Balance	$ 12,794	$ 3,976	$ 12,794	$ 3,976