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Non-Qualified Retirement, Savings and Investment Plans (Details)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
USD ($)
plan
Sep. 30, 2016
USD ($)
Sep. 30, 2017
USD ($)
plan
Sep. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Compensation Related Costs [Abstract]          
Number of non-qualified retirement savings and investment plans | plan 2   2    
Deferred compensation liability, current and long-term $ 26.0   $ 26.0   $ 24.7
Increase (decrease) in compensation expense 0.9 $ 0.9 2.6 $ 1.4  
Diversified investments held in trust 21.4   21.4   $ 19.1
Assets held in trust 1.6   1.6    
Investment gains (losses) $ 0.8 $ 0.8 $ 2.2 $ 1.0