Loans Receivable, Net - Additional Information (Detail)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average risk rating on loan exposure	2.6	2.5
Impaired loans	$ 0	$ 0
Loans held	10,668,677,000	9,756,734,000
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	767,875,000	$ 523,529,000
Joint Venture [Member] | Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	$ 146,100,000