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Long-term Debt (Details) - USD ($)
6 Months Ended
Jun. 30, 2016
Dec. 31, 2015
Nov. 10, 2014
New revolving credit facility      
Long-term debt      
Maximum amount of the Credit Facility     $ 1,500,000,000
Outstanding balance $ 760,000,000 $ 620,000,000  
Weighted average interest rate (as a percent) 1.96% 1.92%  
Letter of credit      
Long-term debt      
Current borrowing capacity     $ 150,000,000
Outstanding balance $ 0 $ 0  
Letter of credit | Minimum      
Long-term debt      
Commitment fees on the unused portion (as a percent) 0.25%    
Letter of credit | Maximum      
Long-term debt      
Commitment fees on the unused portion (as a percent) 0.375%