Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2017	Apr. 30, 2017	Jun. 30, 2015
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, cash on hand, and liquid investments with original maturities of three months or less.
Expected loss	$ 0
Provision for loan losses	$ 0
Walker and Dunlop [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage		15.00%
General Electrical Capital Corporation [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchase price of Loan Portfolio			$ 4,700,000,000