Business Segments - Summary of Results (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Business segments
Net interest income (expense)	$ 716	$ 787	$ 1,466	$ 1,561
Noninterest income (expense)	711	809	1,443	1,421
Total revenue	1,427	1,596	2,909	2,982
Noninterest expense	1,274	1,178	2,485	2,379
(Reversal of) provision for credit losses	(174)	361	(349)	831
Income (loss) before income taxes and including noncontrolling interests	327	57	773	(228)
Income tax expense (benefit)	19	39	89	64
Net income (loss) including noncontrolling interests	308	18	684	(292)
Deduct: net loss from noncontrolling interests	3	4	6	8
Net Income Attributable to MUAH	311	22	690	(284)
Total assets, end of period	165,297	167,350	165,297	167,350	$ 167,846
Operating segments | Regional Bank
Business segments
Net interest income (expense)	519	500	1,017	1,045
Noninterest income (expense)	113	97	229	226
Total revenue	632	597	1,246	1,271
Noninterest expense	541	502	1,072	1,026
(Reversal of) provision for credit losses	(137)	317	(282)	684
Income (loss) before income taxes and including noncontrolling interests	228	(222)	456	(439)
Income tax expense (benefit)	53	(65)	106	(104)
Net income (loss) including noncontrolling interests	175	(157)	350	(335)
Deduct: net loss from noncontrolling interests	0	0	0	0
Net Income Attributable to MUAH	175	(157)	350	(335)
Total assets, end of period	61,821	71,941	61,821	71,941
Operating segments | Global Corporate & Investment Banking - U.S.
Business segments
Net interest income (expense)	131	95	259	220
Noninterest income (expense)	109	116	214	207
Total revenue	240	211	473	427
Noninterest expense	118	115	241	237
(Reversal of) provision for credit losses	(32)	41	(53)	151
Income (loss) before income taxes and including noncontrolling interests	154	55	285	39
Income tax expense (benefit)	27	(3)	62	(22)
Net income (loss) including noncontrolling interests	127	58	223	61
Deduct: net loss from noncontrolling interests	0	0	0	0
Net Income Attributable to MUAH	127	58	223	61
Total assets, end of period	24,201	21,819	24,201	21,819
Operating segments | Transaction Banking
Business segments
Net interest income (expense)	46	46	89	92
Noninterest income (expense)	7	9	13	18
Total revenue	53	55	102	110
Noninterest expense	47	44	89	84
(Reversal of) provision for credit losses	0	0	(1)	0
Income (loss) before income taxes and including noncontrolling interests	6	11	14	26
Income tax expense (benefit)	2	0	4	3
Net income (loss) including noncontrolling interests	4	11	10	23
Deduct: net loss from noncontrolling interests	0	0	0	0
Net Income Attributable to MUAH	4	11	10	23
Total assets, end of period	289	251	289	251
Operating segments | MUSA
Business segments
Net interest income (expense)	75	96	150	162
Noninterest income (expense)	80	187	181	238
Total revenue	155	283	331	400
Noninterest expense	113	150	241	266
(Reversal of) provision for credit losses	0	0	0	0
Income (loss) before income taxes and including noncontrolling interests	42	133	90	134
Income tax expense (benefit)	10	34	22	34
Net income (loss) including noncontrolling interests	32	99	68	100
Deduct: net loss from noncontrolling interests	0	0	0	0
Net Income Attributable to MUAH	32	99	68	100
Total assets, end of period	29,265	27,881	29,265	27,881
Segment Reconciling Items
Business segments
Net interest income (expense)	(55)	50	(49)	42
Noninterest income (expense)	402	400	806	732
Total revenue	347	450	757	774
Noninterest expense	455	367	842	766
(Reversal of) provision for credit losses	(5)	3	(13)	(4)
Income (loss) before income taxes and including noncontrolling interests	(103)	80	(72)	12
Income tax expense (benefit)	(73)	73	(105)	153
Net income (loss) including noncontrolling interests	(30)	7	33	(141)
Deduct: net loss from noncontrolling interests	3	4	6	8
Net Income Attributable to MUAH	(27)	11	39	(133)
Total assets, end of period	$ 49,721	$ 45,458	$ 49,721	$ 45,458