LOANS - Consumer Loans, Delinquencies and Non-Accrual Details (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) category	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) category	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Loans receivable
Number of loan categories | category	2		2
Loans, net of unearned income	$ 691,743		$ 691,743		$ 684,196
Loans at fair value	3,856		3,856		3,223
Consumer
Loans receivable
Loans, current	319,297		319,297		323,436
Loans, net of unearned income	326,038		326,038		330,487
Total non-accrual	2,185		2,185		2,226
90 days past due and accruing	2,332		$ 2,332		$ 2,548
Loans less than this number of days past due are considered current			30 days		30 days
Loans at fair value	18		$ 18		$ 20
Number of days past due, non-accrual status			60 days
Unearned income	745		$ 745		708
Loans sold and/or reclassified to held-for-sale	100	$ 300	2,400	$ 3,000
Consumer | Less than or equal to 80%
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	49,691		49,691		51,844
Consumer | 80% but less than or equal to 100%
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	3,607		3,607		3,761
Consumer | Greater than 100%
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	373		373		587
Consumer | Less than 680
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	38,972		38,972		40,279
Consumer | 680 to 760
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	76,101		76,101		77,963
Consumer | Greater than 760
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	79,640		79,640		83,437
Consumer | Government-guaranteed
Loans receivable
Loans, past due	587		587		786
Consumer | 30 to 89 Days Past Due
Loans receivable
Loans, past due	3,343		3,343		3,264
Consumer | 90 days past due
Loans receivable
Loans, past due	2,811		2,811		3,001
Consumer | Residential first mortgages
Loans receivable
Loans at fair value	18		$ 18		$ 20
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	41,117		$ 41,117		$ 42,379
Consumer | Residential first mortgages | 80% but less than or equal to 100%
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	2,705		2,705		2,474
Consumer | Residential first mortgages | Greater than 100%
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	106		106		197
Consumer | Residential first mortgages | Less than 680
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	3,924		3,924		4,530
Consumer | Residential first mortgages | 680 to 760
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	13,484		13,484		13,848
Consumer | Residential first mortgages | Greater than 760
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	25,671		25,671		26,546
Consumer | Home equity loans | Less than or equal to 80%
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	8,574		8,574		9,465
Consumer | Home equity loans | 80% but less than or equal to 100%
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	902		902		1,287
Consumer | Home equity loans | Greater than 100%
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	267		267		390
Consumer | Home equity loans | Less than 680
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	2,107		2,107		2,438
Consumer | Home equity loans | 680 to 760
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	3,756		3,756		4,296
Consumer | Home equity loans | Greater than 760
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	3,880		3,880		4,471
Consumer | Credit cards | Less than 680
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	32,350		32,350		32,686
Consumer | Credit cards | 680 to 760
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	57,837		57,837		58,722
Consumer | Credit cards | Greater than 760
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	49,110		49,110		51,299
Consumer | Installment and other revolving | Less than 680
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	591		591		625
Consumer | Installment and other revolving | 680 to 760
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	1,024		1,024		1,097
Consumer | Installment and other revolving | Greater than 760
Financing Receivable
Purchased credit impaired in accordance with ASC 310-30	979		$ 979		1,121
Consumer | Commercial banking loans
Loans receivable
Number of days past due, non-accrual status			90 days
Consumer | In North America offices
Loans receivable
Loans, current	206,529		$ 206,529		211,251
Loans, net of unearned income	211,710		211,710		216,694
Total non-accrual	1,168		1,168		1,241
90 days past due and accruing	2,101		2,101		2,313
Consumer | In North America offices | Government-guaranteed
Loans receivable
Loans, past due	587		587		786
Consumer | In North America offices | 30 to 89 Days Past Due
Loans receivable
Loans, past due	2,425		2,425		2,331
Consumer | In North America offices | 90 days past due
Loans receivable
Loans, past due	2,169		2,169		2,326
Consumer | In North America offices | Residential first mortgages
Loans receivable
Loans, current	45,126		45,126		45,953
Loans, net of unearned income	46,337		46,337		47,412
Total non-accrual	558		558		583
90 days past due and accruing	371		371		549
Mortgage loans in process of foreclosure	100		100		100
Consumer | In North America offices | Residential first mortgages | Government-guaranteed
Loans receivable
Loans, past due	587		587		786
Consumer | In North America offices | Residential first mortgages | 30 to 89 Days Past Due
Loans receivable
Loans, past due	409		409		420
Consumer | In North America offices | Residential first mortgages | 30 to 89 Days Past Due | Government-guaranteed
Loans receivable
Loans, past due	200		200		200
Consumer | In North America offices | Residential first mortgages | 90 days past due
Loans receivable
Loans, past due	215		215		253
Consumer | In North America offices | Residential first mortgages | 90 days past due | Government-guaranteed
Loans receivable
Loans, past due	400		400		600
Consumer | In North America offices | Home equity loans
Loans receivable
Loans, current	9,513		9,513		11,135
Loans, net of unearned income	9,850		9,850		11,543
Total non-accrual	433		433		527
90 days past due and accruing	0		0		0
Home equity loans in process of foreclosure	100		100		100
Consumer | In North America offices | Home equity loans | Government-guaranteed
Loans receivable
Loans, past due	0		0		0
Consumer | In North America offices | Home equity loans | 30 to 89 Days Past Due
Loans receivable
Loans, past due	149		149		161
Consumer | In North America offices | Home equity loans | 90 days past due
Loans receivable
Loans, past due	188		188		247
Consumer | In North America offices | Credit cards
Loans receivable
Loans, current	138,009		138,009		141,106
Loans, net of unearned income	141,482		141,482		144,557
Total non-accrual	0		0		0
90 days past due and accruing	1,730		1,730		1,764
Consumer | In North America offices | Credit cards | Government-guaranteed
Loans receivable
Loans, past due	0		0		0
Consumer | In North America offices | Credit cards | 30 to 89 Days Past Due
Loans receivable
Loans, past due	1,743		1,743		1,687
Consumer | In North America offices | Credit cards | 90 days past due
Loans receivable
Loans, past due	1,730		1,730		1,764
Consumer | In North America offices | Installment and other revolving
Loans receivable
Loans, current	3,305		3,305		3,395
Loans, net of unearned income	3,361		3,361		3,454
Total non-accrual	18		18		22
90 days past due and accruing	0		0		0
Consumer | In North America offices | Installment and other revolving | Government-guaranteed
Loans receivable
Loans, past due	0		0		0
Consumer | In North America offices | Installment and other revolving | 30 to 89 Days Past Due
Loans receivable
Loans, past due	41		41		43
Consumer | In North America offices | Installment and other revolving | 90 days past due
Loans receivable
Loans, past due	15		15		16
Consumer | In North America offices | Commercial banking loans
Loans receivable
Loans, current	10,576		10,576		9,662
Loans, net of unearned income	10,680		10,680		9,728
Total non-accrual	159		159		109
90 days past due and accruing	0		0		0
Consumer | In North America offices | Commercial banking loans | Government-guaranteed
Loans receivable
Loans, past due	0		0		0
Consumer | In North America offices | Commercial banking loans | 30 to 89 Days Past Due
Loans receivable
Loans, past due	83		83		20
Consumer | In North America offices | Commercial banking loans | 90 days past due
Loans receivable
Loans, past due	21		21		46
Consumer | In offices outside North America
Loans receivable
Loans, current	112,768		112,768		112,185
Loans, net of unearned income	114,328		114,328		113,793
Total non-accrual	1,017		1,017		985
90 days past due and accruing	231		231		235
Consumer | In offices outside North America | Government-guaranteed
Loans receivable
Loans, past due	0		0		0
Consumer | In offices outside North America | 30 to 89 Days Past Due
Loans receivable
Loans, past due	918		918		933
Consumer | In offices outside North America | 90 days past due
Loans receivable
Loans, past due	642		642		675
Consumer | In offices outside North America | Residential first mortgages
Loans receivable
Loans, current	36,289		36,289		35,624
Loans, net of unearned income	36,644		36,644		35,972
Total non-accrual	392		392		383
90 days past due and accruing	0		0		0
Consumer | In offices outside North America | Residential first mortgages | Government-guaranteed
Loans receivable
Loans, past due	0		0		0
Consumer | In offices outside North America | Residential first mortgages | 30 to 89 Days Past Due
Loans receivable
Loans, past due	219		219		203
Consumer | In offices outside North America | Residential first mortgages | 90 days past due
Loans receivable
Loans, past due	136		136		145
Consumer | In offices outside North America | Credit cards
Loans receivable
Loans, current	23,542		23,542		24,131
Loans, net of unearned income	24,300		24,300		24,926
Total non-accrual	288		288		312
90 days past due and accruing	231		231		235
Consumer | In offices outside North America | Credit cards | Government-guaranteed
Loans receivable
Loans, past due	0		0		0
Consumer | In offices outside North America | Credit cards | 30 to 89 Days Past Due
Loans receivable
Loans, past due	410		410		425
Consumer | In offices outside North America | Credit cards | 90 days past due
Loans receivable
Loans, past due	348		348		370
Consumer | In offices outside North America | Installment and other revolving
Loans receivable
Loans, current	26,297		26,297		25,773
Loans, net of unearned income	26,639		26,639		26,134
Total non-accrual	125		125		152
90 days past due and accruing	0		0		0
Consumer | In offices outside North America | Installment and other revolving | Government-guaranteed
Loans receivable
Loans, past due	0		0		0
Consumer | In offices outside North America | Installment and other revolving | 30 to 89 Days Past Due
Loans receivable
Loans, past due	237		237		254
Consumer | In offices outside North America | Installment and other revolving | 90 days past due
Loans receivable
Loans, past due	105		105		107
Consumer | In offices outside North America | Commercial banking loans
Loans receivable
Loans, current	26,640		26,640		26,657
Loans, net of unearned income	26,745		26,745		26,761
Total non-accrual	212		212		138
90 days past due and accruing	0		0		0
Consumer | In offices outside North America | Commercial banking loans | Government-guaranteed
Loans receivable
Loans, past due	0		0		0
Consumer | In offices outside North America | Commercial banking loans | 30 to 89 Days Past Due
Loans receivable
Loans, past due	52		52		51
Consumer | In offices outside North America | Commercial banking loans | 90 days past due
Loans receivable
Loans, past due	$ 53		$ 53		$ 53