Other Comprehensive Income (Loss) (Components of AOCI) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,358,980	$ 1,298,661
Current period other comprehensive income (loss)	7,252	(9,047)
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	4,350	0
Ending Balance	1,373,816	1,304,886
Unrealized gains (losses) on available for sale debt securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(9,605)	(3,292)
Current period other comprehensive income (loss)	7,578	(9,639)
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	0	(184)
Ending Balance	(2,027)	(13,115)
Post-retirement obligations [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(3,625)	(4,846)
Current period other comprehensive income (loss)	(12)	62
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	0	0
Ending Balance	(3,637)	(4,784)
Unrealized gains (losses) on derivatives (cash flow hedges) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	894	673
Current period other comprehensive income (loss)	(314)	530
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	0	0
Ending Balance	580	1,203
Accumulated Other Comprehensive Income (Loss) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(12,336)	(7,465)
Current period other comprehensive income (loss)	7,252	(9,047)
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	0	(184)
Ending Balance	$ (5,084)	$ (16,696)