Business Segments - Summary of Results (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017		Dec. 31, 2017
Business segments
Net interest income (expense)	$ 825	[1]	$ 794	[1]	$ 1,650		$ 1,589
Noninterest income (expense)	596	[1]	489	[1]	978		977
Total revenue	1,421	[1]	1,283	[1]	2,628		2,566
Noninterest expense	1,083	[1]	957	[1]	2,167		1,963
(Reversal of) provision for credit losses	(19)	[1]	(22)	[1]	(21)		(52)
Income before income taxes and including noncontrolling interests	357	[1]	348	[1]	482		655
Income tax expense (benefit)	28	[1]	63	[1]	(14)		146
Net income including noncontrolling interests	329	[1]	285	[1]	496		509
Deduct: net loss from noncontrolling interests	15	[1]	10	[1]	14		15
Net Income Attributable to MUAH	344	[1]	295	[1]	510		524
Total assets, end of period	160,373	[1]	150,556	[1]	160,373	[1]	150,556	[1]	$ 154,550
Operating segments | Regional Bank
Business segments
Net interest income (expense)	540		510		1,067		1,011
Noninterest income (expense)	116		110		227		218
Total revenue	656	[1]	620	[1]	1,294		1,229
Noninterest expense	532		497		1,034		996
(Reversal of) provision for credit losses	(5)		18		7		20
Income before income taxes and including noncontrolling interests	129	[1]	105	[1]	253		213
Income tax expense (benefit)	26		29		51		56
Net income including noncontrolling interests	103	[1]	76	[1]	202		157
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	103	[1]	76	[1]	202		157
Total assets, end of period	70,186		64,403		70,186		64,403
Operating segments | U.S. Wholesale & Investment Banking
Business segments
Net interest income (expense)	101		112		203		226
Noninterest income (expense)	98		95		180		205
Total revenue	199	[1]	207	[1]	383		431
Noninterest expense	103		96		204		200
(Reversal of) provision for credit losses	(5)		(33)		(24)		(42)
Income before income taxes and including noncontrolling interests	101	[1]	144	[1]	203		273
Income tax expense (benefit)	(4)		35		136		70
Net income including noncontrolling interests	105	[1]	109	[1]	67		203
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	105	[1]	109	[1]	67		203
Total assets, end of period	19,740		22,054		19,740		22,054
Operating segments | Transaction Banking
Business segments
Net interest income (expense)	67		60		131		115
Noninterest income (expense)	15		15		29		29
Total revenue	82	[1]	75	[1]	160		144
Noninterest expense	65		56		121		115
(Reversal of) provision for credit losses	0		0		(1)		0
Income before income taxes and including noncontrolling interests	17	[1]	19	[1]	40		29
Income tax expense (benefit)	4		8		11		12
Net income including noncontrolling interests	13	[1]	11	[1]	29		17
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	13	[1]	11	[1]	29		17
Total assets, end of period	991		1,082		991		1,082
Operating segments | MUSA
Business segments
Net interest income (expense)	51		57		105		116
Noninterest income (expense)	77		88		165		172
Total revenue	128	[1]	145	[1]	270		288
Noninterest expense	116		112		232		217
(Reversal of) provision for credit losses	0		0		0		0
Income before income taxes and including noncontrolling interests	12	[1]	33	[1]	38		71
Income tax expense (benefit)	3		13		9		27
Net income including noncontrolling interests	9	[1]	20	[1]	29		44
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	9	[1]	20	[1]	29		44
Total assets, end of period	32,904		30,439		32,904		30,439
Segment Reconciling Items
Business segments
Net interest income (expense)	66		55		144		121
Noninterest income (expense)	290		181		377		353
Total revenue	356	[1]	236	[1]	521		474
Noninterest expense	267		196		576		435
(Reversal of) provision for credit losses	(9)		(7)		(3)		(30)
Income before income taxes and including noncontrolling interests	98	[1]	47	[1]	(52)		69
Income tax expense (benefit)	(1)		(22)		(221)		(19)
Net income including noncontrolling interests	99	[1]	69	[1]	169		88
Deduct: net loss from noncontrolling interests	15		10		14		15
Net Income Attributable to MUAH	114	[1]	79	[1]	183		103
Total assets, end of period	$ 36,552		$ 32,578		$ 36,552		$ 32,578

[1] As of and for the Three Months Ended June 30, 2018:(Dollars in millions) Regional Bank U.S. Wholesale & Investment Banking Transaction Banking MUSA Other MUFG Americas Holdings CorporationResults of operations Net interest income $540 $101 $67 $51 $66 $825Noninterest income 116 98 15 77 290 596Total revenue 656 199 82 128 356 1,421Noninterest expense 532 103 65 116 267 1,083(Reversal of) provision for credit losses (5) (5) — — (9) (19)Income (loss) before income taxes and including noncontrolling interests 129 1011712 98357Income tax expense (benefit) (1) 26 (4) 4 3 (1) 28Net income (loss) including noncontrolling interests 103 105 13 9 99 329Deduct: net (income) loss from noncontrolling interests — — — — 15 15Net income (loss) attributable to MUAH $103 $105 $13 $9 $114 $344 Total assets, end of period $70,186 $19,740 $991 $32,904 $36,552 $160,373