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Related Party Transactions - Pension and Retirement Savings Plans (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Related Party Transactions [Abstract]        
Health and life insurance costs $ 0.9 $ 0.9 $ 1.7 $ 1.7
Defined contribution benefit plan costs $ 0.3 $ 0.4 $ 0.6 $ 0.7