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Fair Value Measurements and Derivative Instruments - Credit Features (Details)
$ in Millions
1 Months Ended 3 Months Ended
Apr. 30, 2017
Mar. 31, 2017
USD ($)
derivative
Derivative Instruments    
Number of derivatives matured | derivative   2
Financial instruments pledged as collateral | $   $ 8.5
Moody's, Baa3 Rating    
Derivative Instruments    
Credit ratings for senior debt   Baa3
Moody's, Ba1 Rating    
Derivative Instruments    
Credit ratings for senior debt   Ba1
Standard & Poor's, BBB- Rating | Minimum    
Derivative Instruments    
Credit ratings for senior debt   BBB-
Standard & Poor's, BB Rating    
Derivative Instruments    
Credit ratings for senior debt   BB+
Subsequent Event | Moody's, Baa3 Rating    
Derivative Instruments    
Credit ratings for senior debt Baa3