VARIABLE INTEREST ENTITIES - Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Variable Interest Entity [Line Items]
Assets	$ 35,049	$ 34,615	$ 33,695
Liabilities	30,207	29,732
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Assets	150	154
Liabilities	123	136
Maximum Exposure to Loss	149	153
Variable Interest Entity, Primary Beneficiary [Member] | Trust Preferred Securities [Member]
Variable Interest Entity [Line Items]
Assets	1	1
Liabilities	66	66
Maximum Exposure to Loss	0	0
Variable Interest Entity, Primary Beneficiary [Member] | Affordable Housing Tax Credit Partnerships [Member]
Variable Interest Entity [Line Items]
Assets	117	120
Liabilities	47	60
Maximum Exposure to Loss	117	120
Variable Interest Entity, Primary Beneficiary [Member] | Other Investments [Member]
Variable Interest Entity [Line Items]
Assets	32	33
Liabilities	10	10
Maximum Exposure to Loss	$ 32	$ 33