Accumulated Other Comprehensive Income (Loss) - Summary (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at beginning of period			$ 655,947		$ 655,947
Other comprehensive income before reclassifications		$ 3,210		$ 3,278	6,443	$ 6,569
Amounts reclassified from accumulated other comprehensive loss		(1,608)		(1,646)	(3,217)	(3,293)
Net current-period other comprehensive income		1,602		1,632	3,226	3,276
Reclassification of stranded tax effects related to the Tax Act					(16,847)
Balance at end of period		605,709			605,709
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at beginning of period		(93,633)	(78,410)	(87,989)	(78,410)	(89,633)
Reclassification of stranded tax effects related to the Tax Act	[1]				16,847
Balance at end of period		(92,031)	(93,633)	(86,357)	(92,031)	(86,357)
Pension and Other Postretirement Benefits Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at beginning of period		(88,852)	(73,629)	(83,208)	(73,629)	(84,852)
Other comprehensive income before reclassifications		3,210		3,278	6,443	6,569
Amounts reclassified from accumulated other comprehensive loss		(1,608)		(1,646)	(3,217)	(3,293)
Net current-period other comprehensive income		1,602		1,632	3,226	3,276
Reclassification of stranded tax effects related to the Tax Act					(16,847)
Balance at end of period		(87,250)	(88,852)	(81,576)	(87,250)	(81,576)
Foreign Currency Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at beginning of period		(4,781)	(4,781)	(4,781)	(4,781)	(4,781)
Other comprehensive income before reclassifications		0		0	0	0
Amounts reclassified from accumulated other comprehensive loss		0		0	0	0
Net current-period other comprehensive income		0		0	0	0
Reclassification of stranded tax effects related to the Tax Act					0
Balance at end of period		$ (4,781)	$ (4,781)	$ (4,781)	$ (4,781)	$ (4,781)

[1] Refer to Note 1 - Recently Adopted Accounting Pronouncements within our Notes to unaudited Condensed Consolidated Financial Statements for additional details on the adoption of ASU No. 2018-02 during the first quarter of 2018.