Loans Receivable, Net - Additional Information (Detail)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average risk rating on loan exposure	2.7	2.7
Impaired loans	$ 0	$ 0
Loans held	15,053,372,000	14,740,844,000
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	2,256,289,000	2,206,740,000
Joint Venture [Member] | Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	$ 343,400,000	$ 334,600,000