Investments in Real Estate-Related Securities and Loans - Schedule of Company's Affiliate CMBS Positions (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost Basis	$ 3,430,549		$ 3,430,549		$ 2,281,416
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	1,057,625		1,057,625		919,392
Interest Income	12,290	$ 8,771	24,370	$ 15,795
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	164,770		164,770		163,404
Interest Income	2,090	762	4,187	1,448
Commercial Mortgage Backed Securities Collateralized by Properties and Loans Owned by Blackstone-advised Investment Vehicles [Member]
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost Basis	1,222,395		1,222,395		$ 1,082,796
Interest Income	$ 14,380	$ 9,533	$ 28,557	$ 17,243