Borrowings - Narrative (Details) - Line of Credit - USD ($)	1 Months Ended		6 Months Ended	12 Months Ended
	Dec. 31, 2017	Oct. 31, 2017	Jun. 30, 2019	Dec. 31, 2017	Dec. 31, 2018	Nov. 30, 2017
Colony NorthStar, Inc.
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 15,000,000.0	$ 35,000,000			$ 35,000,000.0
Expiration term (in years)		1 year
Line of credit extension term (in months)		6 months
Proceeds drawn under line of credit				$ 25,000,000
Key Bank
Line of Credit Facility [Line Items]
Maximum borrowing capacity					$ 25,000,000.0
Credit facility term (in years)	3 years
LIBOR | Colony NorthStar, Inc.
Line of Credit Facility [Line Items]
Interest rate (percentage)			3.50%
Minimum | LIBOR | Key Bank
Line of Credit Facility [Line Items]
Interest rate (percentage)	2.50%
Maximum | LIBOR | Key Bank
Line of Credit Facility [Line Items]
Interest rate (percentage)	3.50%