Business Segments - Summary of Results (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2017	[1]	Sep. 30, 2016	[2]	Sep. 30, 2017	[3]	Sep. 30, 2016	[4]	Dec. 31, 2016
Business segments
Net interest income (expense)	$ 816		$ 773		$ 2,405		$ 2,251
Noninterest income (expense)	515		570		1,492		1,609
Total revenue	1,331		1,343		3,897		3,860
Noninterest expense	982		952		2,945		2,826
(Reversal of) provision for credit losses	18		73		(34)		196
Income before income taxes and including noncontrolling interests	331		318		986		838
Income tax expense (benefit)	109		97		255		244
Net income including noncontrolling interests	222		221		731		594
Deduct: net loss from noncontrolling interests	10		39		25		62
Net Income Attributable to MUAH	232		260		756		656
Total assets, end of period	154,852	[3]	151,099	[4]	154,852	[1]	151,099	[2]	$ 148,144
Operating segments | Regional Bank
Business segments
Net interest income (expense)	511		481		1,514		1,427
Noninterest income (expense)	113		118		341		344
Total revenue	624		599		1,855		1,771
Noninterest expense	500		459		1,505		1,355
(Reversal of) provision for credit losses	20		23		40		17
Income before income taxes and including noncontrolling interests	104		117		310		399
Income tax expense (benefit)	22		29		68		104
Net income including noncontrolling interests	82		88		242		295
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	82		88		242		295
Total assets, end of period	66,152	[3]	63,343	[4]	66,152	[1]	63,343	[2]
Operating segments | U.S. Wholesale & Investment Banking
Business segments
Net interest income (expense)	110		132		337		422
Noninterest income (expense)	88		109		265		277
Total revenue	198		241		602		699
Noninterest expense	93		96		281		296
(Reversal of) provision for credit losses	(5)		19		(53)		148
Income before income taxes and including noncontrolling interests	110		126		374		255
Income tax expense (benefit)	16		31		56		36
Net income including noncontrolling interests	94		95		318		219
Deduct: net loss from noncontrolling interests	0		0		0		(1)
Net Income Attributable to MUAH	94		95		318		218
Total assets, end of period	21,853	[3]	25,646	[4]	21,853	[1]	25,646	[2]
Operating segments | Transaction Banking
Business segments
Net interest income (expense)	146		125		421		350
Noninterest income (expense)	45		39		124		130
Total revenue	191		164		545		480
Noninterest expense	114		110		348		338
(Reversal of) provision for credit losses	(1)		1		(1)		0
Income before income taxes and including noncontrolling interests	78		53		198		142
Income tax expense (benefit)	30		21		78		56
Net income including noncontrolling interests	48		32		120		86
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	48		32		120		86
Total assets, end of period	1,728	[3]	1,907	[4]	1,728	[1]	1,907	[2]
Operating segments | MUSA
Business segments
Net interest income (expense)	61		46		178		106
Noninterest income (expense)	100		96		272		255
Total revenue	161		142		450		361
Noninterest expense	119		99		336		278
(Reversal of) provision for credit losses	0		0		0		0
Income before income taxes and including noncontrolling interests	42		43		114		83
Income tax expense (benefit)	16		17		45		33
Net income including noncontrolling interests	26		26		69		50
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	26		26		69		50
Total assets, end of period	32,538	[3]	30,526	[4]	32,538	[1]	30,526	[2]
Segment Reconciling Items
Business segments
Net interest income (expense)	(12)		(11)		(45)		(54)
Noninterest income (expense)	169		208		490		603
Total revenue	157		197		445		549
Noninterest expense	156		188		475		559
(Reversal of) provision for credit losses	4		30		(20)		31
Income before income taxes and including noncontrolling interests	(3)		(21)		(10)		(41)
Income tax expense (benefit)	25		(1)		8		15
Net income including noncontrolling interests	(28)		(20)		(18)		(56)
Deduct: net loss from noncontrolling interests	10		39		25		63
Net Income Attributable to MUAH	(18)		19		7		7
Total assets, end of period	$ 32,581	[3]	$ 29,677	[4]	$ 32,581	[1]	$ 29,677	[2]

[1] The transferred IHC entities are not measured using a "market view" perspective.

[2] The transferred IHC entities are not measured using a "market view" perspective.

[3] The transferred IHC entities are not measured using a "market view" perspective.

[4] The transferred IHC entities are not measured using a "market view" perspective.