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