Dividend Reinvestment and Stock Purchase Plan (Details) - USD ($) $ in Millions	9 Months Ended		55 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015
Dividend Reinvestment and Stock Purchase Plan
Authorized common shares to be issued	26,000,000
Number of common shares issued	7,565,432	1,290,872	12,656,940
Amount raised from shares issued	$ 360.9	$ 56.6	$ 579.5
Number of common shares issued under the waiver approval process	7,413,207	1,135,897
Amount raised from share issued under the waiver approval process	$ 353.7	$ 50.0