Annual report pursuant to Section 13 and 15(d)

Investments (Details)

v3.8.0.1
Investments (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule of Investments [Line Items]      
Amortized Cost $ 0.8 $ 35.8  
Fair Value 0.8 35.8  
Redemptions of Available-For-Sale Securities 35.0 10.7 $ 109.8
Proceeds from Sale of Available-For-Sale Securities 0.0 0.0 9.5
Proceeds from Dividends Received 0.4 0.0 0.0
Commercial Paper [Member]      
Schedule of Investments [Line Items]      
Amortized Cost   35.0  
Fair Value 0.0 35.0  
U.S. Treasury Bills [Member]      
Schedule of Investments [Line Items]      
Amortized Cost 0.8 0.8  
Fair Value 0.8 0.8  
TN-BC Holdings, LLC [Member]      
Schedule of Investments [Line Items]      
Payments to Acquire Interest in Joint Venture 2.1    
Mercury Finance [Member]      
Schedule of Investments [Line Items]      
Equity Contributions to Other Existing Joint Ventures $ 1.6 $ 1.0 $ 0.6