Equity - Dividend Reinvestment and Stock Purchase Plan (Details) - USD ($) $ in Thousands	3 Months Ended		85 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018
Dividend Reinvestment and Stock Purchase Plan
Proceeds from dividend reinvestment and stock purchase plan	$ 2,346	$ 56,991
Common stock
Dividend Reinvestment and Stock Purchase Plan
Authorized common shares to be issued	26,000,000
Number of common shares issued	45,933	966,557	14,109,475
Proceeds from dividend reinvestment and stock purchase plan	$ 2,300	$ 57,000	$ 664,200
Number of common shares issued under the waiver approval process		927,695
Amount raised from share issued under the waiver approval process		$ 54,700