Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated other comprehensive loss
Accumulated other comprehensive loss	$ (99,005)	$ (104,233)	$ (81,897)
Gain (Loss) recognized in OCI, Net	(2,060)	(26,353)
Gain (Loss) reclassified from AOCI to income, Net	7,288	4,017
Defined benefit pension item
Accumulated other comprehensive loss
Accumulated other comprehensive (loss) income, Gross, Beginning balance	(1,020)	(998)
Accumulated other comprehensive (loss) income, Income tax (benefit), Beginning balance	384	402
Accumulated other comprehensive loss	(479)	(636)	(596)
Gain (Loss) recognized in OCI, Gross	(516)	(510)
Gain (Loss) recognized in OCI, Income tax (benefit)	194	167
Gain (Loss) recognized in OCI, Net	(322)	(343)
Gain (Loss) reclassified from AOCI to income, Gross	768	488
Gain (Loss) reclassified from AOCI to income, Income tax (benefit)	(289)	(185)
Gain (Loss) reclassified from AOCI to income, Net	479	303
Accumulated other comprehensive (loss) income, Gross, Ending balance	(768)	(1,020)
Accumulated other comprehensive (loss) income, Income tax (benefit), Ending balance	289	384
Accumulated Defined Benefit Plans Adjustment
Accumulated other comprehensive loss
Accumulated other comprehensive (loss) income, Gross, Beginning balance	(169,291)	(138,133)
Accumulated other comprehensive (loss) income, Income tax (benefit), Beginning balance	65,124	55,028
Accumulated other comprehensive loss	(97,358)	(104,167)	(83,105)
Gain (Loss) recognized in OCI, Gross	0	(37,043)
Gain (Loss) recognized in OCI, Income tax (benefit)	0	12,267
Gain (Loss) recognized in OCI, Net	0	(24,776)
Gain (Loss) reclassified from AOCI to income, Gross	7,572	5,885
Gain (Loss) reclassified from AOCI to income, Income tax (benefit)	(763)	(2,171)
Gain (Loss) reclassified from AOCI to income, Net	6,809	3,714
Accumulated other comprehensive (loss) income, Gross, Ending balance	(161,719)	(169,291)
Accumulated other comprehensive (loss) income, Income tax (benefit), Ending balance	64,361	65,124
Unrealized gains and losses on cash flow hedges
Accumulated other comprehensive loss
Accumulated other comprehensive (loss) income, Gross, Beginning balance	245	949
Accumulated other comprehensive (loss) income, Income tax (benefit), Beginning balance	325	855
Accumulated other comprehensive loss	(1,168)	570	$ 1,804
Gain (Loss) recognized in OCI, Gross	(1,524)	(704)
Gain (Loss) recognized in OCI, Income tax (benefit)	(214)	(530)
Gain (Loss) recognized in OCI, Net	(1,738)	(1,234)
Gain (Loss) reclassified from AOCI to income, Gross	0	0
Gain (Loss) reclassified from AOCI to income, Income tax (benefit)	0	0
Gain (Loss) reclassified from AOCI to income, Net	0	0
Accumulated other comprehensive (loss) income, Gross, Ending balance	(1,279)	245
Accumulated other comprehensive (loss) income, Income tax (benefit), Ending balance	$ 111	$ 325