Concentration of Credit Risk (Schedule of Concentration Of Credit Risk) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Concentration Risk [Line Items]    
Revenues $ 339.2  
Credit Concentration Risk    
Concentration Risk [Line Items]    
Revenues $ 48.9 $ 47.6
Concentration Risk, Percentage 14.40% 13.80%