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Investments (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Interest Rate Derivative Assets, at Fair Value $ 4.8 $ 7.6
Short-term investments    
Fair Value 33.5 81.4
Long-term investments    
Cost Basis 22.4 96.8
Certificates of deposit    
Short-term investments    
Cost Basis 33.5 81.4
Unrealized Gain 0.0 0.0
Fair Value 33.5 81.4
Assets held in rabbi trusts    
Long-term investments    
Cost Basis 6.5 8.3
Unrealized Gain (0.2) 1.1
Fair Value $ 6.3 $ 9.4