Other Comprehensive Income/Loss (Components of Accumulated Other Comprehensive Loss, Net of Related Tax Effects) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 1,663,448	$ 1,580,070
Other Comprehensive Income	4,410	2,491
Balance at end of period	1,681,801	1,603,290
Net unrealized gain on securities available-for-sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	2,827	4,686
Other Comprehensive Income	3,985	1,993
Balance at end of period	6,812	6,679
Net actuarial loss on pension plans and other postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(58,396)	(67,476)
Other Comprehensive Income	397	469
Balance at end of period	(57,999)	(67,007)
Prior service cost on pension plans and other postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(3,048)	(3,161)
Other Comprehensive Income	28	29
Balance at end of period	(3,020)	(3,132)
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(58,617)	(65,951)
Other Comprehensive Income	4,410	2,491
Balance at end of period	$ (54,207)	$ (63,460)