Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	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 date at 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.
Expected loss	$ 0
Borrower Escrows	450,800,000	$ 320,000,000.0
Restricted cash	0	$ 0
Loans receivable, net	16,400,000,000
Accounting Standards Update 2016-13 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
CECL reserve	$ 17,600,000
CECL per share	$ 0.13
Percentage of CECL reserve on outstanding balance	0.11%
Walker and Dunlop [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage			15.00%