Principles of Consolidation and Basis of Presentation New Accounting Pronouncements and Changes in Accounting Principles (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Nonoperating Income (Expense)	$ (2,040)	$ (545)
Labor and benefits	183,716	167,138
ASU 2017-07 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Nonoperating Income (Expense)		1,600
Labor and benefits		$ 1,600
AOCI Attributable to Parent [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
United States Tax Cuts and Jobs Act 2017, Reclassification from AOCI to Retained Earnings	(2,970)
AOCI Attributable to Parent [Member] | Adjustments for New Accounting Principle, Early Adoption [Member] | ASU 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
United States Tax Cuts and Jobs Act 2017, Reclassification from AOCI to Retained Earnings	3,000
Retained Earnings [Member] | Adjustments for New Accounting Principle, Early Adoption [Member] | ASU 2018-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
United States Tax Cuts and Jobs Act 2017, Reclassification from AOCI to Retained Earnings	$ (3,000)