Business Segments - Summary of Results (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Business segments
Net interest income (expense)	$ 759	$ 754	$ 2,320	$ 2,314
Noninterest income (expense)	767	717	2,188	1,998
Total revenue	1,526	1,471	4,508	4,312
Noninterest expense	1,523	2,770	3,902	5,094
Provision for credit losses	41	95	872	189
Income before income taxes and including noncontrolling interests	(38)	(1,394)	(266)	(971)
Income tax expense (benefit)	(155)	(18)	(91)	30
Net (loss) income including noncontrolling interests	117	(1,376)	(175)	(1,001)
Deduct: net loss from noncontrolling interests	4	4	12	12
Net Income Attributable to MUAH	121	(1,372)	(163)	(989)
Total assets, end of period	164,029	173,233	164,029	173,233	$ 170,810
Operating segments | Regional Bank
Business segments
Net interest income (expense)	528	582	1,573	1,727
Noninterest income (expense)	110	127	336	375
Total revenue	638	709	1,909	2,102
Noninterest expense	497	532	1,523	1,608
Provision for credit losses	52	72	736	155
Income before income taxes and including noncontrolling interests	89	105	(350)	339
Income tax expense (benefit)	21	19	(83)	66
Net (loss) income including noncontrolling interests	68	86	(267)	273
Deduct: net loss from noncontrolling interests	0	0	0	0
Net Income Attributable to MUAH	68	86	(267)	273
Total assets, end of period	69,317	75,446	69,317	75,446
Operating segments | U.S. Wholesale Banking and Investment Banking [Member]
Business segments
Net interest income (expense)			325
Noninterest income (expense)			360
Noninterest expense			389
Provision for credit losses			140
Income tax expense (benefit)			(2)
Deduct: net loss from noncontrolling interests			0
Total assets, end of period	22,936		22,936
Operating segments | Global Corporate & Investment Banking - U.S.
Business segments
Net interest income (expense)	105	116		365
Noninterest income (expense)	153	92		268
Total revenue	258	208	685	633
Noninterest expense	126	129		375
Provision for credit losses	(11)	24		39
Income before income taxes and including noncontrolling interests	143	55	156	219
Income tax expense (benefit)	25	(4)		12
Net (loss) income including noncontrolling interests	118	59	158	207
Deduct: net loss from noncontrolling interests	0	0		0
Net Income Attributable to MUAH	118	59	158	207
Total assets, end of period	22,936	20,964	22,936	20,964
Operating segments | Transaction Banking
Business segments
Net interest income (expense)	50	45	142	149
Noninterest income (expense)	10	10	28	30
Total revenue	60	55	170	179
Noninterest expense	46	43	130	146
Provision for credit losses	0	0	0	(3)
Income before income taxes and including noncontrolling interests	14	12	40	36
Income tax expense (benefit)	4	9	7	15
Net (loss) income including noncontrolling interests	10	3	33	21
Deduct: net loss from noncontrolling interests	0	0	0	0
Net Income Attributable to MUAH	10	3	33	21
Total assets, end of period	250	653	250	653
Operating segments | MUSA
Business segments
Net interest income (expense)	74	39	236	111
Noninterest income (expense)	110	111	348	289
Total revenue	184	150	584	400
Noninterest expense	131	119	397	355
Provision for credit losses	0	0	0	0
Income before income taxes and including noncontrolling interests	53	31	187	45
Income tax expense (benefit)	13	7	47	9
Net (loss) income including noncontrolling interests	40	24	140	36
Deduct: net loss from noncontrolling interests	0	0	0	0
Net Income Attributable to MUAH	40	24	140	36
Total assets, end of period	28,407	35,199	28,407	35,199
Segment Reconciling Items
Business segments
Net interest income (expense)	2	(28)	44	(38)
Noninterest income (expense)	384	377	1,116	1,036
Total revenue	386	349	1,160	998
Noninterest expense	723	1,947	1,463	2,610
Provision for credit losses	0	(1)	(4)	(2)
Income before income taxes and including noncontrolling interests	(337)	(1,597)	(299)	(1,610)
Income tax expense (benefit)	(218)	(49)	(60)	(72)
Net (loss) income including noncontrolling interests	(119)	(1,548)	(239)	(1,538)
Deduct: net loss from noncontrolling interests	4	4	12	12
Net Income Attributable to MUAH	(115)	(1,544)	(227)	(1,526)
Total assets, end of period	$ 43,119	$ 40,971	$ 43,119	$ 40,971