Issuances of Common Stock - Dividend Reinvestment and Stock Purchase Plan (Details) - USD ($) $ in Thousands	9 Months Ended		21 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018
Dividend Reinvestment and Stock Purchase Plan
Proceeds from dividend reinvestment and stock purchase plan	$ 6,966	$ 67,813
Common stock
Dividend Reinvestment and Stock Purchase Plan
Authorized common shares to be issued	26,000,000
Number of common shares issued	131,072	1,155,883	14,194,614
Proceeds from dividend reinvestment and stock purchase plan	$ 7,000	$ 67,800	$ 668,800
Number of common shares issued under the waiver approval process		927,695
Amount raised from share issued under the waiver approval process		$ 54,700