Issuances of Common Stock - Dividend Reinvestment and Stock Purchase Plan (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		103 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019
Dividend Reinvestment and Stock Purchase Plan
Gross proceeds			$ 6,259	$ 6,966
Common stock | Dividend Reinvestment and Stock Purchase Plan
Dividend Reinvestment and Stock Purchase Plan
Authorized common shares to be issued (in shares)			26,000,000
Shares of common stock issued (in shares)	29,801	38,011	89,219	131,072	14,319,029
Gross proceeds	$ 2,100	$ 2,200	$ 6,300	$ 7,000	$ 677,200