Other Comprehensive Income/Loss - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	$ 1,663,448	$ 1,580,070	$ 1,519,513
Other Comprehensive (Loss) Income	1,736	7,334	(27,701)
Balance at the end of the period	1,714,073	1,663,448	1,580,070
Accumulated other comprehensive loss
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(58,617)	(65,951)	(38,250)
Other Comprehensive (Loss) Income	1,736	7,334	(27,701)
Balance at the end of the period	(56,881)	(58,617)	(65,951)
Net unrealized gain on securities available-for-sale
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	2,827	4,686
Other Comprehensive (Loss) Income	(566)	(1,859)	9,052
Balance at the end of the period	2,261	2,827	4,686
Net actuarial loss on pension plans and other postretirement benefits
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(58,396)	(67,476)
Other Comprehensive (Loss) Income	2,189	9,080	(36,876)
Balance at the end of the period	(56,207)	(58,396)	(67,476)
Prior service cost on pension plans and other postretirement benefits
AOCI Attributable to Parent, Net of Tax
Balance at the beginning of the period	(3,048)	(3,161)
Other Comprehensive (Loss) Income	113	113
Balance at the end of the period	$ (2,935)	$ (3,048)	$ (3,161)