Equity - Dividend Reinvestment and Stock Purchase Plan (Details) - USD ($) $ in Millions	12 Months Ended		70 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016
Equity
Authorized common shares to be issued	26,000,000
Number of common shares issued	170,027	7,608,354	12,869,889
Amount raised from shares issued	$ 10.3	$ 363.0	$ 591.9
Number of common shares issued under the waiver approval process		7,413,207
Amount raised from share issued under the waiver approval process		$ 353.7