ALLOWANCE FOR CREDIT LOSSES - Allowance for Loan Losses Roll Forward by Segment (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Allowance for credit losses
Charge-offs	$ (2,479,000,000)	$ (2,345,000,000)
Recoveries	371,000,000	397,000,000
Replenishment of net charge-offs	2,108,000,000	1,948,000,000
Net reserve builds (releases) for loans	4,112,000,000	67,000,000
Net specific reserve builds (releases)	224,000,000	(71,000,000)
Initial allowance for credit losses on newly purchased credit deteriorated assets during the period	4,000,000	0
Other	(483,000,000)	18,000,000
Allowance for credit losses on loans
Collectively evaluated	19,464,000,000		$ 11,293,000,000
Individually evaluated	1,367,000,000		1,489,000,000
Total allowance for credit losses on loans	20,841,000,000	12,329,000,000	12,783,000,000	$ 12,315,000,000
Loans, net of unearned income
Collectively evaluated	709,929,000,000		688,338,000,000
Individually evaluated	6,963,000,000		6,932,000,000
Held at fair value	3,999,000,000		4,085,000,000
Loans, net of unearned income	721,020,000,000		699,483,000,000
Available for sale debt securities, allowance for credit loss	0
Receivables Acquired with Deteriorated Credit Quality
Allowance for credit losses on loans
Purchased credit deteriorated	10,000,000		1,000,000
Loans, net of unearned income
Purchased credit deteriorated	129,000,000		128,000,000
Corporate
Allowance for credit losses
Charge-offs	(138,000,000)	(100,000,000)
Recoveries	11,000,000	21,000,000
Replenishment of net charge-offs	127,000,000	79,000,000
Net reserve builds (releases) for loans	1,268,000,000	4,000,000
Net specific reserve builds (releases)	48,000,000	(79,000,000)
Initial allowance for credit losses on newly purchased credit deteriorated assets during the period	0	0
Other	(30,000,000)	(5,000,000)
Allowance for credit losses on loans
Collectively evaluated	3,168,000,000		2,587,000,000
Individually evaluated	283,000,000		299,000,000
Total allowance for credit losses on loans	3,451,000,000	2,731,000,000	2,886,000,000	2,811,000,000
Loans, net of unearned income
Collectively evaluated	426,125,000,000		383,828,000,000
Individually evaluated	2,484,000,000		2,040,000,000
Held at fair value	3,981,000,000		4,067,000,000
Loans, net of unearned income	432,590,000,000		389,935,000,000
Corporate | Receivables Acquired with Deteriorated Credit Quality
Allowance for credit losses on loans
Purchased credit deteriorated	0		0
Loans, net of unearned income
Purchased credit deteriorated	0		0
Consumer
Allowance for credit losses
Charge-offs	(2,341,000,000)	(2,245,000,000)
Recoveries	360,000,000	376,000,000
Replenishment of net charge-offs	1,981,000,000	1,869,000,000
Net reserve builds (releases) for loans	2,844,000,000	63,000,000
Net specific reserve builds (releases)	176,000,000	8,000,000
Initial allowance for credit losses on newly purchased credit deteriorated assets during the period	4,000,000	0
Other	(453,000,000)	23,000,000
Allowance for credit losses on loans
Collectively evaluated	16,296,000,000		8,706,000,000
Individually evaluated	1,084,000,000		1,190,000,000
Total allowance for credit losses on loans	17,390,000,000	$ 9,598,000,000	9,897,000,000	$ 9,504,000,000
Loans, net of unearned income
Collectively evaluated	283,804,000,000		304,510,000,000
Individually evaluated	4,479,000,000		4,892,000,000
Held at fair value	18,000,000		18,000,000
Loans, net of unearned income	288,430,000,000		309,548,000,000
Consumer | Receivables Acquired with Deteriorated Credit Quality
Allowance for credit losses on loans
Purchased credit deteriorated	10,000,000		1,000,000
Loans, net of unearned income
Purchased credit deteriorated	$ 129,000,000		128,000,000
Cumulative Effect, Period of Adoption, Adjustment
Allowance for credit losses on loans
Total allowance for credit losses on loans			4,201,000,000
Cumulative Effect, Period of Adoption, Adjustment | Corporate
Allowance for credit losses on loans
Total allowance for credit losses on loans			(721,000,000)
Cumulative Effect, Period of Adoption, Adjustment | Consumer
Allowance for credit losses on loans
Total allowance for credit losses on loans			$ 4,922,000,000