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Investments (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Short-term investments    
Fair Value $ 64.1 $ 39.9
Long-term investments    
Cost Basis 94.7 85.2
Certificates of deposit    
Short-term investments    
Cost Basis 64.1 39.9
Unrealized Gain 0.0 0.0
Fair Value 64.1 39.9
Assets held in rabbi trusts    
Long-term investments    
Cost Basis 7.9 9.3
Unrealized Gain 0.6 0.0
Fair Value $ 8.5 $ 9.3