Stockholders' Equity - Issuances of Stock under Equity Distribution Agreements (Details) - USD ($) $ / shares in Units, $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Common Class A | Common Stock
Class Of Stock [Line Items]
Shares issued		2,074,205	4,472,083
Common Class A | Common Stock | Common Equity Distribution Agreements
Class Of Stock [Line Items]
Shares issued		2,074,205	4,472,083
Weighted average public offering price		$ 10.33	$ 13.88
Net proceeds	[1]	$ 21,065	$ 61,213
Series B Preferred Stock | Preferred Stock
Class Of Stock [Line Items]
Shares issued		42,865	303,291
Series B Preferred Stock | Preferred Stock | Series B Preferred Equity Distribution Agreement
Class Of Stock [Line Items]
Shares issued		42,865	168,291
Weighted average public offering price		$ 24.78	$ 24.95
Net proceeds	[1]	$ 1,030	$ 4,090

[1] Net of selling commissions and expenses.