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Investments (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Short-term investments:    
Short-term investments $ 14.5 $ 81.4
Long-term investments:    
Assets held in a rabbi trust 96.9 96.8
Assets held in rabbi trusts [Member]    
Long-term investments:    
Assets held in a rabbi trust 6.7 8.3
Long-term investments, unrealized gain 0.8 1.1
Certificates of deposit    
Short-term investments:    
Investments, Cost Basis 14.5 81.4
Short-term investments, unrealized gain 0.0 0.0
Short-term investments | Certificates of deposit | Fair Value, Measurements, Recurring [Member]    
Short-term investments:    
Fair value of assets held under trust 14.5 81.4
Short-term investments | Certificates of deposit | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]    
Short-term investments:    
Fair value of assets held under trust 14.5 81.4
Other liabilities | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Assets held in rabbi trusts [Member]    
Long-term investments:    
Fair Value Of Assets Held Under Trust $ 7.5 $ 9.4