Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Apr. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage			85.00%
Income accrual, description	Income accrual is generally suspended for loans at the earlier of the dateat which payments become 90 days past due or when, in the opinion of our Manager, recovery of income and principal becomes doubtful
Cash and cash equivalents, description	Cash and cash equivalents represent cash held in banks and liquid investments with original maturities of three months or less
Borrower Escrows	$ 384,600	$ 450,800
Restricted cash	0	0
Cumulative-effect adjustment to retained earnings	(829,284)	$ (592,548)
Increase in CECL reserve	(173,549)
Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase in CECL reserve	69,700
Level 3 [Member] | Fair Value, Nonrecurring [Member] | Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and lease receivable	$ 338,700
Level 3 [Member] | Minimum [Member] | Measurement Input, Cap Rate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Capitalisation rate of loans receivable	4.25%
Level 3 [Member] | Maximum [Member] | Measurement Input, Cap Rate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Capitalisation rate of loans receivable	4.80%
Accounting Standards Update 2016-13 [Member] | Adoption of ASU 2016-13 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Cumulative-effect adjustment to retained earnings	$ 17,700
Walker and Dunlop [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage			15.00%