Loans Receivable, Net - Additional Information (Detail)	Mar. 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	12,136,883,000	11,093,608,000
Risk Rating 4 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held		20,552,000
Risk Rating 4 [Member] | Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net Book Value		20,500,000
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	1,489,938,000	940,980,000
Joint Venture [Member] | Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	$ 155,300,000	$ 182,200,000