Dividend Reinvestment and Stock Purchase Plan (Details) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended 49 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2015
Dividend Reinvestment and Stock Purchase Plan      
Authorized common shares to be issued 11,000,000    
Number of common shares issued 2,023,276 55,870 7,114,784
Amount raised from shares issued $ 102.2 $ 2.3 $ 320.8
Number of common shares issued under the waiver approval process 1,980,994    
Amount raised from share issued under the waiver approval process $ 100.0