SIGNIFICANT ACCOUNTING POLICIES - Mortgage Loans on Real Estate (Details) - USD ($) $ in Millions	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity	$ (11,643)		$ (11,565)	$ (12,359)	$ (19,395)
Retained Earnings
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity	$ 795		(2,145)	$ (5,039)	$ (8,860)
Accounting Standards Update 2016-13 | Cumulative effect of period of adoption, adjusted balance | Retained Earnings
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity		$ 40	$ 36