Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended	6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2020 USD ($) Countries	Dec. 31, 2019 USD ($)	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.
Borrower Escrows	$ 350,400,000	$ 350,400,000	$ 450,800,000
Restricted cash	0	$ 0	0
Number Of Countries | Countries		200
Cumulative-effect adjustment to retained earnings	(820,783,000)	$ (820,783,000)	$ (592,548,000)
Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase in CECL reserve	69,700,000
Level 3 [Member] | Fair Value, Nonrecurring [Member] | Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and lease receivable	$ 334,200,000	$ 334,200,000
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%	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%	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	$ 17,700,000	$ 17,700,000
Walker and Dunlop [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage				15.00%