Long-Term Debt Unused Borrowing Capacity (Details) - Second Amended and Restated Senior Secured Syndicated Credit Facility Agreement [Member] $ in Thousands	Mar. 31, 2016 USD ($)
Revolving credit facility [Member]
Line of Credit Facility [Line Items]
Total available borrowing capacity	$ 625,000
Outstanding borrowings	57,937
Unused borrowing capacity	561,940
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Outstanding letter of credit guarantees	$ 5,123