Loans Receivable, Net - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) SecurityLoan	Dec. 31, 2019 USD ($) SecurityLoan	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	120	128
Net Book Value	$ 16,572,715	$ 16,164,801	$ 14,191,200	$ 10,056,732
Total Loan Exposure	17,454,621	16,965,864
CECL reserve	(173,549)	0
Net book value of restructured loan	$ 16,399,166	$ 16,164,801	$ 14,191,200
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	6
Net Book Value	$ 777,163	$ 376,379
Total Loan Exposure	$ 778,283	$ 378,427
Risk Rating 2 [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	17	30
Net Book Value	$ 2,513,848	$ 3,481,123
Total Loan Exposure	$ 2,528,835	$ 3,504,972
Risk Rating 3 [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	79	89
Net Book Value	$ 9,911,914	$ 12,137,963
Total Loan Exposure	$ 10,763,496	$ 12,912,722
Risk Rating 4 [Member]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of Loans | SecurityLoan	14	3
Net Book Value	$ 3,032,593	$ 169,336
Total Loan Exposure	$ 3,045,309	$ 169,743
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
Net Book Value	$ 337,197
Total Loan Exposure	$ 338,698