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Investments (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2016
Dec. 31, 2015
Short-term investments:    
Short-term investments $ 26.6 $ 39.9
Long-term Investments 84.0 85.2
Certificates of deposit    
Short-term investments:    
Investments, Cost Basis 26.6 39.9
Available-for-sale Securities, Accumulated Gross Unrealized Gain (Loss), before Tax 0.0 0.0
Assets held in rabbi trusts [Member]    
Short-term investments:    
Long-term Investments 7.7 9.3
Long-term investments:    
Long-term investments, Unrealized Gain 0.4 0.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Certificates of deposit    
Short-term investments:    
Short-term investments 26.6 39.9
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 8.1 9.3
Certificates of deposit | Fair Value, Measurements, Recurring [Member]    
Long-term investments:    
Fair value of assets held under trust $ 26.6 $ 39.9