Long-Term Debt Unused borrowing capacity (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	May. 27, 2014
Line of Credit Facility [Line Items]
Outstanding borrowings	$ 2,220,694	$ 1,603,772
Second Amended and Restated Senior Secured Syndicated Credit Facility Agreement [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Total available borrowing capacity	625,000
Outstanding borrowings	65,760
Unused borrowing capacity	556,246
Second Amended and Restated Senior Secured Syndicated Credit Facility Agreement [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Outstanding letter of credit guarantees	$ 2,994
Prior Credit Agreement [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Total available borrowing capacity		625,000	$ 625,000
Outstanding borrowings		43,187
Unused borrowing capacity		579,175
Prior Credit Agreement [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Outstanding letter of credit guarantees		$ 2,638