Loans Receivable, Net - Additional Information (Detail)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average risk rating on loan exposure	2.7	2.5
Impaired loans	$ 0	$ 0
Loans held	11,093,608,000	9,756,734,000
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	1,228,959,000	$ 819,781,000
Joint Venture [Member] | Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	$ 182,200,000