Stock Transactions - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended
	Aug. 10, 2017	Feb. 28, 2016	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)			0		0
Minimum distribution percentage to qualify for REIT taxation status			90.00%
Distribution percentage required to avoid paying federal income taxes			100.00%
Common stock, dividends, per share per annum, declared (in dollars per share)			$ 1.44	$ 1.44
Distributions payable			$ 6,956	$ 6,100	$ 6,912
Total distributions			15,800	14,000
Cash distributions			12,200	10,600
Common stock issued under DRIP			$ 3,600	$ 3,400
Share repurchase program, period in force		1 year	1 year
Repurchase price percent of NAV per share year one	92.50%		92.50%
Repurchase price percent of NAV per share year two	95.00%		95.00%
Repurchase price percent of NAV per share year three	97.50%		97.50%
Repurchase price percent of NAV per share year four	100.00%		100.00%
NAV per share (in dollars per share)						$ 18.57
Book value (in dollars per share)			$ 16.25
Repurchase limit percent per fiscal semester		2.50%	2.50%
Repurchase limit percent per fiscal year		5.00%	5.00%
Common Stock
Class of Stock [Line Items]
Shares outstanding (in shares)			44,396,346		43,916,815
Distributions payable			$ 5,400		$ 5,400
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)			1,400		1,400
Distributions payable			$ 100		$ 100
Series A Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)			40,514	32,245	40,500		29,249
Distributions payable			$ 1,500		$ 1,500