Loans Receivable, Net - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average risk rating on loan exposure	3.0	2.8
Impaired loans	$ 0	$ 0
Loans held	17,161,035,000	16,965,864,000	$ 16,164,801,000
Net book value of restructured loan	16,250,914,000	16,164,801,000
Troubled debt restructuring under GAAP, for one of our loans	52,800,000
Initial CECL reserve	13,942,000
Increase in CECL reserve	$ 98,752,000
COVID-19 pandemic [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average risk rating on loan exposure	4
Loans held	$ 3,100,000,000
Net book value of restructured loan	112,700,000
Initial CECL reserve	13,900,000
Increase in CECL reserve	98,800,000
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	2,010,565,000	1,642,664,000
Joint Venture [Member] | Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held	$ 731,400,000	$ 670,500,000