Long-Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Debt Disclosure [Line Items]
Line of Credit Facility, Potential Additional Borrowing Capacity	$ 3,000,000
Senior secured revolving credit line	944,663	$ 725,500
Other Long-term Debt	187	213
Total debt	950,708	732,126
Less: Current maturities of long-term debt	5,192	5,883
Long-term debt	945,516	726,243
Line of Credit Facility, Outstanding Amount, USD Borrowings	921,500
Line of Credit Facility, Amount Outstanding, GBP Borrowings	23,200
Foreign
Debt Disclosure [Line Items]
Foreign loans	$ 5,858	$ 6,413