LOANS - Consumer Loan Delinquency (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Non-Accrual Details
Loans, net of unearned income	$ 672,938	$ 667,034
Loans at fair value	4,536	4,374
Consumer
Non-Accrual Details
Loans, current	318,114	326,092
Loans, net of unearned income	325,084	333,656
Total non-accrual	2,575	2,690
90 days past due and accruing	$ 2,547	$ 2,812
Loans less than this number of days past due are considered current	30 days	30 days
Loans at fair value	$ 23	$ 25
Number of days past due, non-accrual status	60 days
Consumer | Less than or equal to 80%
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	$ 53,578	55,029
Consumer | 80% but less than or equal to 100%
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	4,490	4,725
Consumer | Greater than 100%
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	951	1,047
Consumer | Less than 620
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	12,140	12,834
Consumer | ≥ 620 but less than 660
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	14,197	14,914
Consumer | ≥ 660 but less than 720
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	47,749	48,960
Consumer | Equal to or greater than 720
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	115,970	123,343
Consumer | Government-guaranteed
Non-Accrual Details
Loans, past due	1,010	1,225
Consumer | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	3,103	3,304
Consumer | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	2,857	3,035
Consumer | Residential first mortgages
Non-Accrual Details
Loans at fair value	$ 23	$ 25
Number of days past due, non-accrual status	90 days	90 days
Consumer | Residential first mortgages | Less than or equal to 80%
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	$ 42,790	$ 43,626
Consumer | Residential first mortgages | 80% but less than or equal to 100%
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	2,536	2,578
Consumer | Residential first mortgages | Greater than 100%
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	214	247
Consumer | Residential first mortgages | Less than 620
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	1,750	2,100
Consumer | Residential first mortgages | ≥ 620 but less than 660
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	1,746	1,932
Consumer | Residential first mortgages | ≥ 660 but less than 720
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	6,655	6,931
Consumer | Residential first mortgages | Equal to or greater than 720
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	35,253	35,334
Consumer | Home equity loans | Less than or equal to 80%
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	10,788	11,403
Consumer | Home equity loans | 80% but less than or equal to 100%
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	1,954	2,147
Consumer | Home equity loans | Greater than 100%
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	737	800
Consumer | Home equity loans | Less than 620
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	1,075	1,379
Consumer | Home equity loans | ≥ 620 but less than 660
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	937	1,081
Consumer | Home equity loans | ≥ 660 but less than 720
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	3,138	3,446
Consumer | Home equity loans | Equal to or greater than 720
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	8,409	8,530
Consumer | Credit cards | Less than 620
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	9,169	9,079
Consumer | Credit cards | ≥ 620 but less than 660
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	11,285	11,651
Consumer | Credit cards | ≥ 660 but less than 720
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	37,275	37,916
Consumer | Credit cards | Equal to or greater than 720
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	70,598	77,661
Consumer | Installment and other revolving | Less than 620
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	146	276
Consumer | Installment and other revolving | ≥ 620 but less than 660
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	229	250
Consumer | Installment and other revolving | ≥ 660 but less than 720
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	681	667
Consumer | Installment and other revolving | Equal to or greater than 720
Financing Receivable
Purchased credit-impaired in accordance with ASC 310-30	$ 1,710	1,818
Consumer | Commercial banking loans
Non-Accrual Details
Number of days past due, non-accrual status	90 days
Consumer | Total GCB and Corporate/Other— Consumer
Non-Accrual Details
Loans, current	$ 318,113	326,091
Loans, net of unearned income	325,083	333,655
Total non-accrual	2,575	2,690
90 days past due and accruing	2,547	2,812
Consumer | Total GCB and Corporate/Other— Consumer | Government-guaranteed
Non-Accrual Details
Loans, past due	1,010	1,225
Consumer | Total GCB and Corporate/Other— Consumer | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	3,103	3,304
Consumer | Total GCB and Corporate/Other— Consumer | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	2,857	3,035
Consumer | Other
Non-Accrual Details
Loans, current	1	1
Loans, net of unearned income	1	1
Total non-accrual	0	0
90 days past due and accruing	0	0
Consumer | Other | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | Other | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	0	0
Consumer | Other | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	0	0
Consumer | In North America offices
Non-Accrual Details
Loans, current	201,217	211,012
Loans, net of unearned income	206,421	216,899
Total non-accrual	1,500	1,650
90 days past due and accruing	2,291	2,553
Consumer | In North America offices | Government-guaranteed
Non-Accrual Details
Loans, past due	1,010	1,225
Consumer | In North America offices | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	2,036	2,336
Consumer | In North America offices | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	2,158	2,326
Consumer | In North America offices | Residential first mortgages
Non-Accrual Details
Loans, current	46,607	47,366
Loans, net of unearned income	48,238	49,376
Total non-accrual	636	665
90 days past due and accruing	750	941
Mortgage loans in process of foreclosure	100	100
Consumer | In North America offices | Residential first mortgages | Government-guaranteed
Non-Accrual Details
Loans, past due	1,010	1,225
Consumer | In North America offices | Residential first mortgages | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	364	505
Consumer | In North America offices | Residential first mortgages | 30 to 89 Days Past Due | Government-guaranteed
Non-Accrual Details
Loans, past due	200	200
Consumer | In North America offices | Residential first mortgages | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	257	280
Consumer | In North America offices | Residential first mortgages | Equal to greater than 90 days past due | Government-guaranteed
Non-Accrual Details
Loans, past due	800	1,000
Consumer | In North America offices | Home equity loans
Non-Accrual Details
Loans, current	13,476	14,268
Loans, net of unearned income	13,951	14,827
Total non-accrual	691	750
90 days past due and accruing	0	0
Home equity loans in process of foreclosure	100	100
Consumer | In North America offices | Home equity loans | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In North America offices | Home equity loans | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	160	207
Consumer | In North America offices | Home equity loans | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	315	352
Consumer | In North America offices | Credit cards
Non-Accrual Details
Loans, current	128,790	136,588
Loans, net of unearned income	131,778	139,729
Total non-accrual	0	0
90 days past due and accruing	1,528	1,596
Consumer | In North America offices | Credit cards | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In North America offices | Credit cards | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	1,460	1,528
Consumer | In North America offices | Credit cards | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	1,528	1,613
Consumer | In North America offices | Installment and other revolving
Non-Accrual Details
Loans, current	3,333	3,395
Loans, net of unearned income	3,385	3,456
Total non-accrual	21	22
90 days past due and accruing		1
Consumer | In North America offices | Installment and other revolving | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In North America offices | Installment and other revolving | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	38	45
Consumer | In North America offices | Installment and other revolving | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	14	16
Consumer | In North America offices | Commercial banking loans
Non-Accrual Details
Loans, current	9,011	9,395
Loans, net of unearned income	9,069	9,511
Total non-accrual	152	213
90 days past due and accruing	13	15
Consumer | In North America offices | Commercial banking loans | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In North America offices | Commercial banking loans | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	14	51
Consumer | In North America offices | Commercial banking loans | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	44	65
Consumer | In offices outside North America
Non-Accrual Details
Loans, current	116,896	115,079
Loans, net of unearned income	118,662	116,756
Total non-accrual	1,075	1,040
90 days past due and accruing	256	259
Consumer | In offices outside North America | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In offices outside North America | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	1,067	968
Consumer | In offices outside North America | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	699	709
Consumer | In offices outside North America | Residential first mortgages
Non-Accrual Details
Loans, current	37,447	37,062
Loans, net of unearned income	37,844	37,419
Total non-accrual	412	400
90 days past due and accruing	0	0
Consumer | In offices outside North America | Residential first mortgages | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In offices outside North America | Residential first mortgages | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	250	209
Consumer | In offices outside North America | Residential first mortgages | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	147	148
Consumer | In offices outside North America | Credit cards
Non-Accrual Details
Loans, current	24,702	24,934
Loans, net of unearned income	25,512	25,727
Total non-accrual	323	323
90 days past due and accruing	256	259
Consumer | In offices outside North America | Credit cards | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In offices outside North America | Credit cards | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	437	427
Consumer | In offices outside North America | Credit cards | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	373	366
Consumer | In offices outside North America | Installment and other revolving
Non-Accrual Details
Loans, current	26,243	25,634
Loans, net of unearned income	26,670	26,032
Total non-accrual	161	157
90 days past due and accruing	0	0
Consumer | In offices outside North America | Installment and other revolving | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In offices outside North America | Installment and other revolving | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	311	275
Consumer | In offices outside North America | Installment and other revolving | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	116	123
Consumer | In offices outside North America | Commercial banking loans
Non-Accrual Details
Loans, current	28,504	27,449
Loans, net of unearned income	28,636	27,578
Total non-accrual	179	160
90 days past due and accruing	0	0
Consumer | In offices outside North America | Commercial banking loans | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In offices outside North America | Commercial banking loans | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	69	57
Consumer | In offices outside North America | Commercial banking loans | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	$ 63	$ 72