Loans Receivable, Net - Additional Information (Detail)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
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	14,740,844,000	11,093,608,000
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	1,973,084,000	1,228,959,000
Joint Venture [Member] | Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	$ 334,600,000	$ 182,200,000