FAIR VALUE MEASUREMENTS - Estimated Fair Values for All Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financial assets
Assets transferred or pledged to securitization vehicles	$ 4,738,481	$ 6,910,020	[1]
Corporate debt, held for sale	65,622,352	75,652,396
Financial liabilities
Other secured financing	729,555	917,876	[1]
Carrying Value | Level 3
Financial assets
Commercial real estate debt and preferred equity, held for investment	0	1,372,430
Corporate debt, held for investment	1,890,709	2,239,930
Financial liabilities
Mortgages payable	0	426,256
Carrying Value | Level 3 | Corporate Loans
Financial assets
Corporate debt, held for sale	2,113	0
Carrying Value | Level 2
Financial assets
Assets transferred or pledged to securitization vehicles	0	874,349
Financial liabilities
Repurchase agreements	55,475,420	64,825,239
Other secured financing	729,555	917,876
Fair Value | Level 3
Financial assets
Commercial real estate debt and preferred equity, held for investment	0	1,442,071
Corporate debt, held for investment	1,904,287	2,226,045
Financial liabilities
Mortgages payable	0	474,779
Fair Value | Level 3 | Corporate Loans
Financial assets
Corporate debt, held for sale	2,113	0
Fair Value | Level 2
Financial assets
Assets transferred or pledged to securitization vehicles	0	928,732
Financial liabilities
Repurchase agreements	55,475,420	64,825,239
Other secured financing	$ 729,555	$ 917,876

[1] (1)Derived from the audited consolidated financial statements at December 31, 2020.