Principles of Consolidation and Basis of Presentation New Accounting Pronouncements and Changes in Accounting Principles (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in operating income - Labor and benefits expense	$ 179,838	$ 164,222	$ 363,554	$ 331,360
Other income/(loss), net	$ 288	3,196	(1,752)	2,651
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
ASU 2017-07 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in operating income - Labor and benefits expense		1,600		3,200
Other income/(loss), net		$ (1,600)		$ (3,200)
Adjustments for New Accounting Principle, Early Adoption [Member] | AOCI Attributable to Parent [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
Adjustments for New Accounting Principle, Early Adoption [Member] | Retained Earnings [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)