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Investments (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2020
Dec. 31, 2019
Assets held in rabbi trusts [Member]    
Long-term investments:    
Assets held in a rabbi trust $ 5.2 $ 5.5
Long-term investments, unrealized gain 0.5 0.7
Certificates of deposit    
Short-term investments:    
Investments, Cost Basis 9.0 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 9.0 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 9.0 10.0
Assets held in a rabbi trust | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Assets held in rabbi trusts [Member]    
Short-term investments:    
Fair value of assets held under trust $ 5.7 $ 6.2