Stockholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands								1 Months Ended					12 Months Ended			75 Months Ended	87 Months Ended
	Oct. 30, 2019	Apr. 30, 2019	Apr. 16, 2018	Mar. 13, 2018	Mar. 01, 2018	Apr. 01, 2017	Feb. 17, 2017	Dec. 31, 2019	Jul. 31, 2018	May 31, 2018	Jan. 31, 2018	Apr. 30, 2013	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2019
Class of Stock [Line Items]
Common stock, shares outstanding (in shares)								92,356,664					92,356,664	91,963,532		91,963,532	92,356,664
Weighted-average price per share (in usd per share)	$ 17.50	$ 20.25							$ 20.25		$ 21.45		$ 19.14			$ 21.56	$ 20.95
Number of shares repurchased (in shares)	446,830	656,433							155,904		373,967		1,103,263			3,288,256	4,391,519
Stock repurchase, value	$ 7,800	$ 13,300								$ 3,000
Common stock repurchases	$ 7,800								$ 3,200		$ 8,000		$ 21,113	$ 14,202	$ 33,599
DRIP, period of notice to alter agreement							10 days
Common stock issued through distribution reinvestment plan (in shares)													1,500,000	1,700,000
Common stock issued through distribution reinvestment plan													$ 27,210	$ 35,737	$ 61,206
Preferred stock, shares authorized								1,610,000					1,610,000	0		0	1,610,000
Preferred stock, shares issued								1,610,000					1,610,000	0		0	1,610,000
Preferred stock, shares outstanding								1,610,000					1,610,000	0		0	1,610,000
Preferred stock, liquidation preference (in usd per share)								$ 25.00					$ 25.00				$ 25.00
Payments for underwriting expense								$ (1,300)
Payments of additional offering expenses								$ (1,400)
Preferred stock, dividend rate (in usd per share)													$ 1.84375
Preferred stock, dividend rate, percentage													7.375%
Common stock, par value (in usd per share)								$ 0.01					$ 0.01	$ 0.01		$ 0.01	$ 0.01
Distributions declared per share (in usd per share)					$ 0.85	$ 1.45						$ 1.70	$ 0.85	$ 0.95	$ 1.51
Preferred stock, quarterly dividend rate (in usd per share)													0.460938
Tender Offer
Class of Stock [Line Items]
Shares authorized for repurchase (in shares)				2,000,000.0
Weighted-average price per share (in usd per share)				$ 13.15
Reduction in shares authorized for repurchase (in shares)			230,000
Number of shares repurchased (in shares)										229,999
Public Stock Offering
Class of Stock [Line Items]
Preferred stock, shares authorized								1,610,000					1,610,000				1,610,000
Preferred stock, number of shares issued in public offering								1,610,000
Preferred stock, liquidation preference (in usd per share)								$ 25.00					$ 25.00				$ 25.00
Sale of stock, gross proceeds								$ 40,300
Sale of stock, net proceeds								$ 37,600
Delisting
Class of Stock [Line Items]
Preferred stock, dividend rate (in usd per share)													$ 2.34375
Preferred stock, dividend rate, percentage													9.375%
Increase in preferred stock, dividend rate, percentage													2.00%
Maximum
Class of Stock [Line Items]
Preferred stock, shares authorized								50,000,000					50,000,000				50,000,000