Equity - Dividend Reinvestment and Stock Purchase Plan (Details) - USD ($) $ in Thousands	12 Months Ended			82 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
Dividend Reinvestment and Stock Purchase Plan
Proceeds from dividend reinvestment and stock purchase plan	$ 69,931	$ 10,252	$ 363,029
Common stock
Dividend Reinvestment and Stock Purchase Plan
Authorized common shares to be issued	26,000,000
Number of common shares issued	1,193,653	170,027		14,063,542
Proceeds from dividend reinvestment and stock purchase plan	$ 69,900	$ 10,300		$ 661,800
Number of common shares issued under the waiver approval process	927,695
Amount raised from share issued under the waiver approval process	$ 54,700