Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated other comprehensive loss
Accumulated other comprehensive loss	$ (78,960)	$ (93,194)	$ (99,005)
Gain (Loss) recognized in OCI, Net	605	(1,190)
Gain (Loss) reclassified from AOCI to income, Net	13,629	7,001
Defined benefit pension item
Accumulated other comprehensive loss
Accumulated other comprehensive (loss) income, Gross, Beginning balance	116	(768)
Accumulated other comprehensive (loss) income, Income tax (benefit), Beginning balance	(42)	289
Accumulated other comprehensive loss	184	74	(479)
Gain (Loss) recognized in OCI, Gross	264	(56)
Gain (Loss) recognized in OCI, Income tax (benefit)	(96)	20
Gain (Loss) recognized in OCI, Net	168	(36)
Gain (Loss) reclassified from AOCI to income, Gross	(91)	940
Gain (Loss) reclassified from AOCI to income, Income tax (benefit)	33	(351)
Gain (Loss) reclassified from AOCI to income, Net	(58)	589
Accumulated other comprehensive (loss) income, Gross, Ending balance	289	116
Accumulated other comprehensive (loss) income, Income tax (benefit), Ending balance	(105)	(42)
Accumulated Defined Benefit Plans Adjustment
Accumulated other comprehensive loss
Accumulated other comprehensive (loss) income, Gross, Beginning balance	(151,618)	(161,719)
Accumulated other comprehensive (loss) income, Income tax (benefit), Beginning balance	60,672	64,361
Accumulated other comprehensive loss	(77,259)	(90,946)	(97,358)
Gain (Loss) recognized in OCI, Gross	0	0
Gain (Loss) recognized in OCI, Income tax (benefit)	0	0
Gain (Loss) recognized in OCI, Net	0	0
Gain (Loss) reclassified from AOCI to income, Gross	21,522	10,101
Gain (Loss) reclassified from AOCI to income, Income tax (benefit)	(7,835)	(3,689)
Gain (Loss) reclassified from AOCI to income, Net	13,687	6,412
Accumulated other comprehensive (loss) income, Gross, Ending balance	(130,096)	(151,618)
Accumulated other comprehensive (loss) income, Income tax (benefit), Ending balance	52,837	60,672
Unrealized gains and losses on cash flow hedges
Accumulated other comprehensive loss
Accumulated other comprehensive (loss) income, Gross, Beginning balance	(2,414)	(1,279)
Accumulated other comprehensive (loss) income, Income tax (benefit), Beginning balance	92	111
Accumulated other comprehensive loss	(1,885)	(2,322)	$ (1,168)
Gain (Loss) recognized in OCI, Gross	429	(1,135)
Gain (Loss) recognized in OCI, Income tax (benefit)	8	(19)
Gain (Loss) recognized in OCI, Net	437	(1,154)
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	(1,985)	(2,414)
Accumulated other comprehensive (loss) income, Income tax (benefit), Ending balance	$ 100	$ 92