Stockholders' Equity - Distribution Reinvestment Plan (Details) - USD ($) $ / shares in Units, shares in Millions			6 Months Ended	12 Months Ended	105 Months Ended
	Jan. 19, 2016	Feb. 02, 2015	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Number of shares issued (in shares)		108.6
Common Stock
Class of Stock [Line Items]
Number of shares issued (in shares)	173.4
Value of common stock issued	$ 1,700,000,000
Dividend Reinvestment Plan
Class of Stock [Line Items]
Share price (in dollars per share)						$ 7.10	$ 8.50	$ 9.10	$ 8.63
Selling commissions or dealer manager fees paid			$ 0
Notice period served by board of directors to amend or terminate DRP (in days)			10 days
Dividend Reinvestment Plan | Common Stock
Class of Stock [Line Items]
Number of shares issued (in shares)			0.7	4.0	25.7
Value of common stock issued			$ 4,900,000	$ 33,700,000	$ 232,600,000
Initial Distribution Support Agreement | Dividend Reinvestment Plan
Class of Stock [Line Items]
Share price (in dollars per share)			$ 9.50		$ 9.50
Follow-on Distribution Reinvestment Plan
Class of Stock [Line Items]
Share price (in dollars per share)			$ 10.20		$ 10.20
Percentage of offering price			95.00%		95.00%
Follow-on Distribution Reinvestment Plan | Dividend Reinvestment Plan
Class of Stock [Line Items]
Share price (in dollars per share)			$ 9.69		$ 9.69