Investments in Real Estate-Related Securities - Schedule of Company's Affiliate CMBS Positions (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost Basis	$ 1,013,713		$ 1,013,713		$ 623,149
Interest Income	12,527	$ 3,136	29,770	$ 4,326
Commercial Mortgage Backed Securities Collateralized by Properties Owned by Blackstone-advised Investment Vehicles [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost Basis	847,605		847,605		559,616
Interest Income	10,617	2,486	26,412	3,673
Commercial Mortgage Backed Securities Collateralized by Loan Originated by Blackstone-advised Investment Vehicle [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost Basis	166,108		166,108		$ 63,533
Interest Income	$ 1,910	$ 650	$ 3,358	$ 653