Long-term Debt Unused Borrowing Capacity (Details) - Credit Agreement [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Outstanding revolving loans	$ 1,783,423	$ 1,567,882
Revolving credit facility [Member]
Line of Credit Facility [Line Items]
Total available borrowing capacity	625,000
Outstanding revolving loans	164,136
Unused borrowing capacity	458,258
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Outstanding letter of credit guarantees	$ 2,606