Loans Receivable, Net - Additional Information (Detail)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average risk rating on loan exposure	2.8	2.7
Impaired loans	$ 0	$ 0
Loans held	16,965,864,000	14,740,844,000
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	1,642,664,000	2,206,740,000
Joint Venture [Member] | Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	$ 670,500,000	$ 334,600,000