Equity - Dividend Reinvestment and Stock Purchase Plan (Details) - USD ($) $ in Millions	3 Months Ended		73 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017
Equity
Authorized common shares to be issued	26,000,000
Number of common shares issued	966,557	61,458	13,836,446
Amount raised from shares issued	$ 57.0	$ 3.5	$ 650.2
Number of common shares issued under the waiver approval process	927,695
Amount raised from share issued under the waiver approval process	$ 54.7