Investments in Real Estate-Related Securities and Loans - Schedule of Company's Affiliate CMBS Positions (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Cost Basis	$ 3,760,776		$ 3,760,776		$ 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	1,069,935		1,069,935		919,392
Interest Income	13,580	$ 10,617	37,950	$ 26,412
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	162,663		162,663		163,404
Interest Income	2,012	1,910	6,199	3,358
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	1,232,598		1,232,598		$ 1,082,796
Interest Income	$ 15,592	$ 12,527	$ 44,149	$ 29,770