Pension and Other Postretirement Benefit Costs - Amounts Recognized in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Beginning balance	$ (8,284)	$ (6,815)	$ (8,438)	$ (6,951)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax	209	225	418	450
Total reclassifications before tax	209	225	418	450
Tax (benefit) expense	(56)	(88)	(111)	(177)
Total reclassifications for the period	153	137	307	273
Ending balance	$ (8,131)	$ (6,678)	$ (8,131)	$ (6,678)

Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e637-108580

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- Definition

Amount, after reclassification adjustment, of tax (expense) benefit for (increase) decrease in accumulated other comprehensive income for defined benefit plan, attributable to parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount, before tax, after reclassification adjustment, of (increase) decrease in accumulated other comprehensive income for defined benefit plan.

+ References

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Amount, after tax and reclassification adjustment, of (increase) decrease in accumulated other comprehensive income for defined benefit plan.

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Amount, before tax, of reclassification adjustment from accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan.

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-Name Accounting Standards Codification
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