Long-Term Debt Unused Borrowing Capacity (Details) - Second Amended and Restated Senior Secured Syndicated Credit Facility Agreement [Member] $ in Thousands	Jun. 30, 2016 USD ($)
Revolving credit facility [Member]
Line of Credit Facility [Line Items]
Total available borrowing capacity	$ 625,000
Less: Outstanding revolving loans	77,698
Total unused borrowing capacity	542,456
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Less: Outstanding letter of credit guarantees	$ 4,846