Business Segments - Summary of Results (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2019		Sep. 30, 2018		Sep. 30, 2019		Sep. 30, 2018		Dec. 31, 2018
Business segments
Net interest income (expense)	$ 754	[1]	$ 832	[1]	$ 2,314		$ 2,482
Noninterest income (expense)	717	[1]	626	[1]	1,998		1,604
Total revenue	1,471	[1]	1,458	[1]	4,312		4,086
Noninterest expense	2,770	[1]	1,059	[1]	5,094		3,226
(Reversal of) provision for credit losses	95	[1]	64	[1]	189		43
Income before income taxes and including noncontrolling interests	(1,394)	[1]	335	[1]	(971)		817
Income tax expense (benefit)	(18)	[1]	35	[1]	30		21
Net (loss) income including noncontrolling interests	(1,376)	[1]	300	[1]	(1,001)		796
Deduct: net loss from noncontrolling interests	4	[1]	6	[1]	12		20
Net Income Attributable to MUAH	(1,372)	[1]	306	[1]	(989)		816
Total assets, end of period	173,233	[1]	161,036	[1]	173,233	[1]	161,036	[1]	$ 168,100
Operating segments | Regional Bank
Business segments
Net interest income (expense)	582		553		1,727		1,620
Noninterest income (expense)	127		111		375		338
Total revenue	709	[1]	664	[1]	2,102		1,958
Noninterest expense	532		518		1,608		1,552
(Reversal of) provision for credit losses	72		45		155		52
Income before income taxes and including noncontrolling interests	105	[1]	101	[1]	339		354
Income tax expense (benefit)	19		17		66		68
Net (loss) income including noncontrolling interests	86	[1]	84	[1]	273		286
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	86	[1]	84	[1]	273		286
Total assets, end of period	75,446		71,501		75,446		71,501
Operating segments | Global Corporate & Investment Banking - U.S.
Business segments
Net interest income (expense)	104		104		320		307
Noninterest income (expense)	87		103		255		283
Total revenue	191	[1]	207	[1]	575		590
Noninterest expense	116		106		341		310
(Reversal of) provision for credit losses	24		27		39		3
Income before income taxes and including noncontrolling interests	51	[1]	74	[1]	195		277
Income tax expense (benefit)	(5)		(3)		6		133
Net (loss) income including noncontrolling interests	56	[1]	77	[1]	189		144
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	56	[1]	77	[1]	189		144
Total assets, end of period	20,820		19,467		20,820		19,467
Operating segments | Transaction Banking
Business segments
Net interest income (expense)	61		66		198		197
Noninterest income (expense)	14		15		44		44
Total revenue	75	[1]	81	[1]	242		241
Noninterest expense	59		65		188		186
(Reversal of) provision for credit losses	0		(3)		(2)		(4)
Income before income taxes and including noncontrolling interests	16	[1]	19	[1]	56		59
Income tax expense (benefit)	10		5		20		16
Net (loss) income including noncontrolling interests	6	[1]	14	[1]	36		43
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	6	[1]	14	[1]	36		43
Total assets, end of period	971		882		971		882
Operating segments | MUSA
Business segments
Net interest income (expense)	39		52		111		157
Noninterest income (expense)	111		94		289		259
Total revenue	150	[1]	146	[1]	400		416
Noninterest expense	119		121		355		353
(Reversal of) provision for credit losses	0		0		0		0
Income before income taxes and including noncontrolling interests	31	[1]	25	[1]	45		63
Income tax expense (benefit)	7		6		9		15
Net (loss) income including noncontrolling interests	24	[1]	19	[1]	36		48
Deduct: net loss from noncontrolling interests	0		0		0		0
Net Income Attributable to MUAH	24	[1]	19	[1]	36		48
Total assets, end of period	35,199		33,366		35,199		33,366
Segment Reconciling Items
Business segments
Net interest income (expense)	(32)		57		(42)		201
Noninterest income (expense)	378		303		1,035		680
Total revenue	346	[1]	360	[1]	993		881
Noninterest expense	1,944		249		2,602		825
(Reversal of) provision for credit losses	(1)		(5)		(3)		(8)
Income before income taxes and including noncontrolling interests	(1,597)	[1]	116	[1]	(1,606)		64
Income tax expense (benefit)	(49)		10		(71)		(211)
Net (loss) income including noncontrolling interests	(1,548)	[1]	106	[1]	(1,535)		275
Deduct: net loss from noncontrolling interests	4		6		12		20
Net Income Attributable to MUAH	(1,544)	[1]	112	[1]	(1,523)		295
Total assets, end of period	$ 40,797		$ 35,820		$ 40,797		$ 35,820

[1] As of and for the Three Months Ended September 30, 2019:(Dollars in millions) Regional Bank Global Corporate & Investment Banking - U.S. Transaction Banking MUSA Other MUFG Americas Holdings CorporationResults of operations Net interest income $582 $104 $61 $39 $(32) $754Noninterest income 127 87 14 111 378 717Total revenue 709 191 75 150 346 1,471Noninterest expense 532 116 59 119 1,944 2,770(Reversal of) provision for credit losses 72 24 — — (1) 95Income (loss) before income taxes and including noncontrolling interests 105 511631 (1,597)(1,394)Income tax expense (benefit) (1) 19 (5) 10 7 (49) (18)Net income (loss) including noncontrolling interests 86 56 6 24 (1,548) (1,376)Deduct: net (income) loss from noncontrolling interests — — — — 4 4Net income (loss) attributable to MUAH $86 $56 $6 $24 $(1,544) $(1,372) Total assets, end of period $75,446 $20,820 $971 $35,199 $40,797 $173,233