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Fair Value Measurements and Derivative Instruments - Credit Features (Details)
$ in Millions
9 Months Ended
Sep. 30, 2017
derivative
Dec. 31, 2016
USD ($)
derivative
Derivative Instruments    
Number of derivatives matured | derivative 4 2
Posted collateral | $   $ 7.2
Moody's, Baa3 Rating    
Derivative Instruments    
Credit ratings for senior debt Baa3  
Moody's, Ba1 Rating    
Derivative Instruments    
Credit ratings for senior debt Baa3  
Standard & Poor's, BBB- Rating | Minimum    
Derivative Instruments    
Credit ratings for senior debt BBB-  
Standard & Poor's, BB Plus Rating    
Derivative Instruments    
Credit ratings for senior debt BBB-