Investments in Real Estate-Related Securities and Loans - Schedule of Company's Affiliate CMBS Positions (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost Basis	$ 1,067,092		$ 1,082,796
Interest Income	14,523	$ 7,710
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	901,274		919,392
Interest Income	12,426	7,024
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	165,818		$ 163,404
Interest Income	$ 2,097	$ 686