Secured Debt Agreements, Net - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Debt Instrument [Line Items]
Interest expense	$ 82,352,000	$ 84,853,000	$ 160,723,000		$ 189,092,000
Covenants, minimum tangible net worth	$ 3,000,000,000.0		$ 3,000,000,000.0
Covenants, percentage of tangible assets on cash proceeds from equity issuances			75.00%
Covenants, percentage of recourse indebtedness	5.00%		5.00%
Maximum [Member]
Debt Instrument [Line Items]
Covenants, indebtedness to total assets, in percent	83.33%		83.33%
Minimum [Member]
Debt Instrument [Line Items]
Covenants, EBITDA to fixed charges, in percent			1.40%
Covenants, minimum cash liquidity amount	$ 10,000,000.0		$ 10,000,000.0
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Interest expense			618,000	$ 635,000
Amortization of deferred fees and expenses			178,000	188,000
Revolving Credit Facility [Member] | Barclays [Member]
Debt Instrument [Line Items]
Maximum Facility Size	250,000,000.0		250,000,000.0
Credit Facilities [Member]
Debt Instrument [Line Items]
Collateral Assets	13,222,033,000		13,222,033,000	$ 12,040,907,000
Line of credit facility, remaining borrowing capacity	1,100,000,000		1,100,000,000
Credit Facilities Six [Member]
Debt Instrument [Line Items]
Potential	2,100,000,000		2,100,000,000
Collateral Assets	2,600,000,000		2,600,000,000
BXMTCredit Facilities [Member]
Debt Instrument [Line Items]
Additional credit capacity	500,000,000.0
BXMTCredit Facilities [Member] | European Bank [Member]
Debt Instrument [Line Items]
Maximum Facility Size	$ 1,800,000		$ 1,800,000