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Investments (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Assets held in rabbi trusts [Member]      
Long-term investments:      
Assets held in a rabbi trust $ 5.1   $ 5.5
Long-term investments, unrealized gain 0.9 $ 0.7  
Certificates of deposit      
Short-term investments:      
Investments, Cost Basis 6.8   10.0
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 6.8   10.0
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 $ 6.8   $ 10.0