Loans Receivable, Net - Additional Information (Detail)	Sep. 30, 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	12,726,831,000	11,093,608,000
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	1,903,572,000	940,980,000
Joint Venture [Member] | Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	$ 222,700,000	$ 182,200,000