Issuances of Common Stock - Dividend Reinvestment and Stock Purchase Plan (Details) - USD ($) $ in Thousands	12 Months Ended			94 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
Dividend Reinvestment and Stock Purchase Plan
Proceeds from dividend reinvestment and stock purchase plan	$ 9,114	$ 69,931	$ 10,252
Common stock
Dividend Reinvestment and Stock Purchase Plan
Authorized common shares to be issued (in shares)	26,000,000
Number of common shares issued (in shares)	166,268	1,193,653		14,229,810
Proceeds from dividend reinvestment and stock purchase plan	$ 9,100	$ 69,900		$ 670,900
Number of common shares issued under the waiver approval process (in shares)		927,695
Amount raised from share issued under the waiver approval process		$ 54,700