Stock Transactions - Narrative (Details) - USD ($)			9 Months Ended
	Aug. 10, 2017	Feb. 28, 2016	Sep. 30, 2019	Sep. 30, 2018	Sep. 11, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
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			$ 6,456,000	$ 5,834,000				$ 5,834,000
Total distributions			44,100,000	35,000,000
Cash distributions			33,700,000	24,400,000
Common stock issued under DRIP			$ 10,400,000	$ 10,600,000
Share repurchase program, period in force		1 year
Repurchase price percent of NAV per share year one	92.50%
Repurchase price percent of NAV per share year two	95.00%
Repurchase price percent of NAV per share year three	97.50%
Repurchase price percent of NAV per share year four	100.00%
NAV per share (in dollars per share)			$ 18.57
Book value (in dollars per share)			$ 18.56
Repurchase limit percent per fiscal semester		2.50%
Repurchase limit percent per fiscal year		5.00%
Common Stock
Class of Stock [Line Items]
Shares outstanding (in shares)			43,021,064					39,303,710
Outstanding binding purchase commitments			$ 3,800,000					$ 0
Distributions payable			$ 5,100,000					4,700,000
Convertible Preferred Stock
Class of Stock [Line Items]
Outstanding binding purchase commitments								$ 0
Preferred stock, shares outstanding (in shares)			38,501	19,215				29,249			0
Distributions payable			$ 1,400,000					$ 1,100,000
Series C Preferred Stock
Class of Stock [Line Items]
Outstanding binding purchase commitments			$ 5,000,000		$ 5,000,000
Common Stock
Class of Stock [Line Items]
Shares outstanding (in shares)			43,021,064	37,477,393		41,720,045	40,258,666	39,303,710	32,620,659	31,600,114	31,834,072