XML 68 R56.htm IDEA: XBRL DOCUMENT v3.8.0.1
Investments (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Short-term investments:    
Short-term investments $ 24.2 $ 81.4
Long-term investments:    
Assets held in a rabbi trust 97.5 96.8
Assets held in rabbi trusts [Member]    
Long-term investments:    
Assets held in a rabbi trust 8.3 8.3
Long-term investments, unrealized gain 0.9 1.1
Certificates of deposit    
Short-term investments:    
Investments, Cost Basis 24.2 81.4
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 24.2 81.4
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 24.2 81.4
Other liabilities | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Assets held in rabbi trusts [Member]    
Long-term investments:    
Fair Value Of Assets Held Under Trust $ 9.2 $ 9.4