Dividend Reinvestment and Stock Purchase Plan (Details) - USD ($) $ in Millions	6 Months Ended		52 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015
Dividend Reinvestment and Stock Purchase Plan
Authorized common shares to be issued	11,000,000
Number of common shares issued	4,335,580	1,240,305	9,427,088
Amount raised from shares issued	$ 208.3	$ 54.4	$ 426.9
Number of common shares issued under the waiver approval process	4,239,477	1,135,897
Amount raised from share issued under the waiver approval process	$ 203.7	$ 50.0