Dividend Reinvestment and Stock Purchase Plan (Details) - USD ($) $ in Millions	12 Months Ended			58 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015
Dividend Reinvestment and Stock Purchase Plan
Authorized common shares to be issued	26,000,000
Number of common shares issued	7,608,354	3,527,166	1,449,139	12,699,862
Amount raised from shares issued	$ 363.0	$ 158.5	$ 55.6	$ 581.6
Number of common shares issued under the waiver approval process	7,413,207	3,330,556	1,308,490
Amount raised from share issued under the waiver approval process	$ 353.7	$ 150.0	$ 50.0