Long-term Debt Unused Borrowing Capacity (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Outstanding revolving loans	$ 2,256,312	$ 2,273,058
Credit Agreement [Member] | Revolving credit facility [Member]
Line of Credit Facility [Line Items]
Total available borrowing capacity	625,000
Outstanding revolving loans	228,640
Unused borrowing capacity	393,120
Credit Agreement [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Outstanding letter of credit guarantees	$ 3,240