Investments in Real Estate-Related Securities - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Investment	Sep. 30, 2017 USD ($)
Investment [Line Items]
Realized gain (loss) on CMBS investment paid off	$ 28,647,000	$ 4,026,000	$ 59,279,000	$ 7,435,000
Unrealized gain (loss) on investments in real estate related securities	4,900,000	(600,000)	8,800,000	1,000,000
Securities sold	$ 0	$ 0	$ 0	$ 0
Commercial Mortgage Backed Securities Collateralized by Properties Owned by Blackstone-advised Investment Vehicles [Member]
Investment [Line Items]
Number of investments paid off | Investment			3
Cost of trading securities paid off			$ 115,600,000
Realized gain (loss) on CMBS investment paid off			$ 0