Loans Receivable, Net - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) SecurityLoan	Dec. 31, 2020 USD ($) SecurityLoan	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	156	120
Net Book Value	$ 20,406,466	$ 16,572,715
Total Loan Exposure	21,520,167	17,454,621
CECL reserve	(130,388)	(173,549)	$ (128,945)	$ (172,100)	$ (177,026)	$ (178,050)	$ (112,694)	$ 0
Net book value of restructured loan	$ 20,276,078	$ 16,399,166
Risk Rating 1 [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	8	8
Net Book Value	$ 931,330	$ 777,163
Total Loan Exposure	$ 931,831	$ 778,283
Risk Rating 2 [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	27	17
Net Book Value	$ 4,800,884	$ 2,513,848
Total Loan Exposure	$ 4,836,029	$ 2,528,835
Risk Rating 3 [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	110	79
Net Book Value	$ 12,060,623	$ 9,911,914
Total Loan Exposure	$ 13,129,870	$ 10,763,496
Risk Rating 4 [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	9	14
Net Book Value	$ 2,276,394	$ 3,032,593
Total Loan Exposure	$ 2,283,701	$ 3,045,309
Risk Rating 5 [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	2	2
Net Book Value	$ 337,235	$ 337,197
Total Loan Exposure	$ 338,736	$ 338,698