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Investments (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Assets held in rabbi trusts [Member]      
Short-term investments:      
Fair value of assets held under trust $ 6.7   $ 6.6
Long-term investments:      
Assets held in a rabbi trust 5.3   5.2
Long-term investments, unrealized gain 1.4 $ 1.4  
Certificates of deposit      
Short-term investments:      
Investments, Cost Basis 21.3   37.2
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 21.3   37.2
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 $ 21.3   $ 37.2