Other Comprehensive Income/Loss - Components of Accumulated Other Comprehensive Loss, Net of Related Tax Effects (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
AOCI Attributable to Parent, Net of Tax
Balance at beginning of period	$ 1,714,073	$ 1,663,448
Other Comprehensive (Loss) Income	(32)	4,410
Balance at end of period	1,723,964	1,681,801
Net unrealized gain on securities available-for-sale
AOCI Attributable to Parent, Net of Tax
Balance at beginning of period	2,261	2,827
Other Comprehensive (Loss) Income	(432)	3,985
Balance at end of period	1,829	6,812
Net actuarial loss on pension plans and other postretirement benefits
AOCI Attributable to Parent, Net of Tax
Balance at beginning of period	(56,207)	(58,396)
Other Comprehensive (Loss) Income	371	397
Balance at end of period	(55,836)	(57,999)
Prior service cost on pension plans and other postretirement benefits
AOCI Attributable to Parent, Net of Tax
Balance at beginning of period	(2,935)	(3,048)
Other Comprehensive (Loss) Income	29	28
Balance at end of period	(2,906)	(3,020)
Accumulated other comprehensive loss
AOCI Attributable to Parent, Net of Tax
Balance at beginning of period	(56,881)	(58,617)
Other Comprehensive (Loss) Income	(32)	4,410
Balance at end of period	$ (56,913)	$ (54,207)