Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2021	Dec. 31, 2020	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		$ 386.3	$ 384.6
SOFR [Member] | Credit Spread Option [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Credit spread adjustment rate		0.11%
Average compounded secured overnight financing rate		0.03%
USD LIBOR Rate [Member] | Credit Spread Option [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average compounded secured overnight financing rate		0.10%
Sterling Overnight Interbank Average Rate [Member] | Credit Spread Option [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Daily compounded sterling overnight interbank average rate		0.05%
GBP LIBOR Rate [Member] | Credit Spread Option [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Daily compounded sterling overnight interbank average rate		0.08%
Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase in CECL reserve	$ 69.7
Level 3 [Member] | Fair Value, Nonrecurring [Member] | Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and lease receivable		$ 338.7
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%
ASU 2020-06 [Member] | January 1, 2022 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Adjustment to additional paid in capital other one		$ 2.4
Decrease in accumulated deficit		$ 2.0
Walker and Dunlop [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage				15.00%