Other Comprehensive Income/Loss (Components of Accumulated Other Comprehensive Loss, Net of Related Tax Effects) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period			$ 1,663,448	$ 1,580,070
Other Comprehensive Income	$ 851	$ (2,347)	5,261	144
Balance at end of period	1,695,324	1,628,522	1,695,324	1,628,522
Net unrealized gain on securities available-for-sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	6,812	6,679	2,827	4,686
Other Comprehensive Income	507	(2,791)	4,492	(798)
Balance at end of period	7,319	3,888	7,319	3,888
Net actuarial loss on pension plans and other postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(57,999)	(67,007)	(58,396)	(67,476)
Other Comprehensive Income	315	417	712	886
Balance at end of period	(57,684)	(66,590)	(57,684)	(66,590)
Prior service cost on pension plans and other postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(3,020)	(3,132)	(3,048)	(3,161)
Other Comprehensive Income	29	27	57	56
Balance at end of period	(2,991)	(3,105)	(2,991)	(3,105)
Accumulated other comprehensive loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(54,207)	(63,460)	(58,617)	(65,951)
Other Comprehensive Income	851	(2,347)	5,261	144
Balance at end of period	$ (53,356)	$ (65,807)	$ (53,356)	$ (65,807)