Other Comprehensive (Loss) Income (Components of AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,298,661	$ 1,251,781
Current period other comprehensive (loss) income	(9,047)	858
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	0	0
Ending balance	1,304,886	1,267,092
Unrealized gains on securities available for sale [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(3,292)	(510)
Current period other comprehensive (loss) income	(9,639)	771
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	(184)	0
Ending balance	(13,115)	261
Post-retirement obligations [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(4,846)	(3,056)
Current period other comprehensive (loss) income	62	32
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	0	0
Ending balance	(4,784)	(3,024)
Unrealized gains (losses) on derivatives (cash flow hedges) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	673	169
Current period other comprehensive (loss) income	530	55
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	0	0
Ending balance	1,203	224
Accumulated Other Comprehensive (Loss) Income [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(7,465)	(3,397)
Current period other comprehensive (loss) income	(9,047)	858
Reclassification of unrealized gains on equity securities due to the adoption of ASU No. 2016-01	(184)
Ending balance	$ (16,696)	$ (2,539)