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Investments (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Interest Rate Derivative Assets, at Fair Value $ 7.6 $ 8.4
Short-term investments    
Fair Value 81.4 64.1
Long-term investments    
Cost Basis 96.8 94.7
Certificates of deposit    
Short-term investments    
Cost Basis 81.4 64.1
Unrealized Gain 0.0 0.0
Fair Value 81.4 64.1
Assets held in rabbi trusts    
Long-term investments    
Cost Basis 8.3 7.9
Unrealized Gain 1.1 0.6
Fair Value $ 9.4 $ 8.5