Other Comprehensive Income/Loss - Components of Other Comprehensive Income/Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other Comprehensive Income (Loss), After Reclassification Adjustments [Abstract]
Before Tax Amount	$ 2,914	$ 12,308	$ (45,487)
Tax Benefit (Expense)	(1,178)	(4,974)	17,786
Total other comprehensive income (loss), net of tax	1,736	7,334	(27,701)
Net unrealized gain on securities available-for-sale
Other Comprehensive Income (Loss), Before Reclassifications [Abstract]
Before Tax Amount	(864)	(3,048)	14,134
Tax Benefit (Expense)	349	1,232	(4,991)
After Tax Amount	(515)	(1,816)	9,143
Other Comprehensive Income (Loss), Reclassifications Adjustments [Abstract]
Before Tax Amount	(86)	(72)	(141)
Tax Benefit (Expense)	35	29	50
After Tax Amount	(51)	(43)	(91)
Other Comprehensive Income (Loss), After Reclassification Adjustments [Abstract]
Before Tax Amount	(950)	(3,120)	13,993
Tax Benefit (Expense)	384	1,261	(4,941)
Total other comprehensive income (loss), net of tax	(566)	(1,859)	9,052
Net actuarial loss on pension plans and other postretirement benefits
Other Comprehensive Income (Loss), Before Reclassifications [Abstract]
Before Tax Amount	1,284	12,265	(60,583)
Tax Benefit (Expense)	(519)	(4,956)	23,116
After Tax Amount	765	7,309	(37,467)
Other Comprehensive Income (Loss), Reclassifications Adjustments [Abstract]
Before Tax Amount	2,390	2,973	913
Tax Benefit (Expense)	(966)	(1,202)	(322)
After Tax Amount	1,424	1,771	591
Other Comprehensive Income (Loss), After Reclassification Adjustments [Abstract]
Before Tax Amount	3,674	15,238	(59,670)
Tax Benefit (Expense)	(1,485)	(6,158)	22,794
Total other comprehensive income (loss), net of tax	2,189	9,080	(36,876)
Prior service cost on pension plans and other postretirement benefits
Other Comprehensive Income (Loss), Reclassifications Adjustments [Abstract]
Before Tax Amount	190	190	190
Tax Benefit (Expense)	(77)	(77)	(67)
After Tax Amount	113	113	$ 123
Other Comprehensive Income (Loss), After Reclassification Adjustments [Abstract]
Total other comprehensive income (loss), net of tax	$ 113	$ 113