Business Segments - Summary of Results (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019		Jun. 30, 2018		Jun. 30, 2019		Jun. 30, 2018		Dec. 31, 2018
Business segments
Net interest income (expense)	$ 777	[1]	$ 825	[1]	$ 1,560		$ 1,650
Noninterest income (expense)	649	[1]	596	[1]	1,281		978
Total revenue	1,426	[1]	1,421	[1]	2,841		2,628
Noninterest expense	1,154	[1]	1,083	[1]	2,324		2,167
(Reversal of) provision for credit losses	56	[1]	(19)	[1]	94		(21)
Income before income taxes and including noncontrolling interests	216	[1]	357	[1]	423		482
Income tax expense (benefit)	20	[1]	28	[1]	48		(14)
Net income including noncontrolling interests	196	[1]	329	[1]	375		496
Deduct: net loss from noncontrolling interests	3	[1]	15	[1]	8		14
Net Income Attributable to MUAH	199	[1]	344	[1]	383		510
Total assets, end of period	172,010	[1]	160,373	[1]	172,010	[1]	160,373	[1]	$ 168,100
Operating segments | Regional Bank
Business segments
Net interest income (expense)	573		540		1,145		1,067
Noninterest income (expense)	118		116		249		227
Total revenue	691	[1]	656	[1]	1,394		1,294
Noninterest expense	538		532		1,078		1,034
(Reversal of) provision for credit losses	32		(5)		80		7
Income before income taxes and including noncontrolling interests	121	[1]	129	[1]	236		253
Income tax expense (benefit)	20		26		46		51
Net income including noncontrolling interests	101	[1]	103	[1]	190		202
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	101	[1]	103	[1]	190		202
Total assets, end of period	75,858		70,186		75,858		70,186
Operating segments | U.S. Wholesale & Investment Banking
Business segments
Net interest income (expense)	116		101		216		203
Noninterest income (expense)	79		98		167		180
Total revenue	195	[1]	199	[1]	383		383
Noninterest expense	113		103		224		204
(Reversal of) provision for credit losses	27		(5)		21		(24)
Income before income taxes and including noncontrolling interests	55	[1]	101	[1]	138		203
Income tax expense (benefit)	(4)		(4)		9		136
Net income including noncontrolling interests	59	[1]	105	[1]	129		67
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	59	[1]	105	[1]	129		67
Total assets, end of period	20,331		19,740		20,331		19,740
Operating segments | Transaction Banking
Business segments
Net interest income (expense)	69		67		137		131
Noninterest income (expense)	14		15		29		29
Total revenue	83	[1]	82	[1]	166		160
Noninterest expense	57		65		130		121
(Reversal of) provision for credit losses	0		0		(2)		(1)
Income before income taxes and including noncontrolling interests	26	[1]	17	[1]	38		40
Income tax expense (benefit)	7		4		10		11
Net income including noncontrolling interests	19	[1]	13	[1]	28		29
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	19	[1]	13	[1]	28		29
Total assets, end of period	911		991		911		991
Operating segments | MUSA
Business segments
Net interest income (expense)	37		51		72		105
Noninterest income (expense)	85		77		178		165
Total revenue	122	[1]	128	[1]	250		270
Noninterest expense	113		116		236		232
(Reversal of) provision for credit losses	0		0		0		0
Income before income taxes and including noncontrolling interests	9	[1]	12	[1]	14		38
Income tax expense (benefit)	2		3		2		9
Net income including noncontrolling interests	7	[1]	9	[1]	12		29
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	7	[1]	9	[1]	12		29
Total assets, end of period	35,131		32,904		35,131		32,904
Segment Reconciling Items
Business segments
Net interest income (expense)	(18)		66		(10)		144
Noninterest income (expense)	353		290		658		377
Total revenue	335	[1]	356	[1]	648		521
Noninterest expense	333		267		656		576
(Reversal of) provision for credit losses	(3)		(9)		(5)		(3)
Income before income taxes and including noncontrolling interests	5	[1]	98	[1]	(3)		(52)
Income tax expense (benefit)	(5)		(1)		(19)		(221)
Net income including noncontrolling interests	10	[1]	99	[1]	16		169
Deduct: net loss from noncontrolling interests	3		15		8		14
Net Income Attributable to MUAH	13	[1]	114	[1]	24		183
Total assets, end of period	$ 39,779		$ 36,552		$ 39,779		$ 36,552

[1] As of and for the Three Months Ended June 30, 2019:(Dollars in millions) Regional Bank U.S. Wholesale & Investment Banking Transaction Banking MUSA Other MUFG Americas Holdings CorporationResults of operations Net interest income $573 $116 $69 $37 $(18) $777Noninterest income 118 79 14 85 353 649Total revenue 691 195 83 122 335 1,426Noninterest expense 538 113 57 113 333 1,154(Reversal of) provision for credit losses 32 27 — — (3) 56Income (loss) before income taxes and including noncontrolling interests 121 55269 5216Income tax expense (benefit) (1) 20 (4) 7 2 (5) 20Net income (loss) including noncontrolling interests 101 59 19 7 10 196Deduct: net (income) loss from noncontrolling interests — — — — 3 3Net income (loss) attributable to MUAH $101 $59 $19 $7 $13 $199 Total assets, end of period $75,858 $20,331 $911 $35,131 $39,779 $172,010