Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncement, Early Adoption
Other long-term assets	$ 252,854	$ 230,090
Long-term Debt, Excluding Current Maturities, Total	4,027,766	3,578,335
Accrued liabilities	533,044	543,248
Retained earnings	1,423,154	$ 2,033,427
Accounting Standards Update 2016-09
New Accounting Pronouncement, Early Adoption
Decrease in deferred tax liabilities	7,100
Decrease in capital in excess	1,900
Accounting Standards Update 2016-01
New Accounting Pronouncement, Early Adoption
Adjustment to retained earnings	9,100
Adjustment | Accounting Standards Update 2016-09
New Accounting Pronouncement, Early Adoption
Increase in retained earnings	$ 7,100