Equity - Dividend Reinvestment and Stock Purchase Plan (Details) - USD ($) $ in Thousands	6 Months Ended		18 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018
Dividend Reinvestment and Stock Purchase Plan
Proceeds from dividend reinvestment and stock purchase plan	$ 4,806	$ 59,649
Common stock
Dividend Reinvestment and Stock Purchase Plan
Authorized common shares to be issued	26,000,000
Number of common shares issued	93,061	1,013,412	14,156,603
Proceeds from dividend reinvestment and stock purchase plan	$ 4,800	$ 59,600	$ 666,600
Number of common shares issued under the waiver approval process		927,695
Amount raised from share issued under the waiver approval process		$ 54,700