Business Segments (Details 2) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Business segments
Net interest income (expense)	$ 719	$ 763	$ 1,402	$ 1,446
Noninterest income (expense)	385	202	720	383
Total revenue	1,104	965	2,122	1,829
Noninterest expense	843	635	1,692	1,262
Provision for Loan and Lease Losses	26	9	23	(7)
Provision for credit losses	15	6	18	6
Income before income taxes and including noncontrolling interests	246	324	412	561
Income tax expense (benefit)	71	84	105	154
Net Income Including Noncontrolling Interests	175	240	307	407
Deduct: net loss from noncontrolling interests	6	4	11	9
Net Income Attributable to MUAH	181	244	318	416
Total assets, end of period	114,266	108,809	114,266	108,809	$ 113,623
Reconciling Items
Business segments
Net interest income (expense)	(98)	(97)	(196)	(187)
Noninterest income (expense)	(50)	(58)	(102)	(102)
Total revenue	(148)	(155)	(298)	(289)
Noninterest expense	(44)	(38)	(92)	(73)
Provision for Loan and Lease Losses				(19)
Provision for credit losses	2	(7)	5
Income before income taxes and including noncontrolling interests	(106)	(110)	(211)	(197)
Income tax expense (benefit)	(42)	(43)	(84)	(77)
Net Income Including Noncontrolling Interests	(64)	(67)	(127)	(120)
Net Income Attributable to MUAH	(64)	(67)	(127)	(120)
Total assets, end of period	(3,683)	(3,679)	(3,683)	(3,679)
Retail Banking and Wealth Markets | Operating segments
Business segments
Net interest income (expense)	320	362	645	709
Noninterest income (expense)	84	87	170	166
Total revenue	404	449	815	875
Noninterest expense	362	341	715	676
Provision for Loan and Lease Losses				(8)
Provision for credit losses	6	(2)	8
Income before income taxes and including noncontrolling interests	36	110	92	207
Income tax expense (benefit)	14	44	36	81
Net Income Including Noncontrolling Interests	22	66	56	126
Net Income Attributable to MUAH	22	66	56	126
Total assets, end of period	37,155	36,429	37,155	36,429
Commercial Banking | Operating segments
Business segments
Net interest income (expense)	236	277	470	503
Noninterest income (expense)	44	60	91	100
Total revenue	280	337	561	603
Noninterest expense	89	85	184	171
Provision for Loan and Lease Losses				6
Provision for credit losses	20	(10)	33
Income before income taxes and including noncontrolling interests	171	262	344	426
Income tax expense (benefit)	51	87	101	138
Net Income Including Noncontrolling Interests	120	175	243	288
Net Income Attributable to MUAH	120	175	243	288
Total assets, end of period	36,892	33,750	36,892	33,750
Corporate Banking | Operating segments
Business segments
Net interest income (expense)				90
Noninterest income (expense)				45
Total revenue			191	135
Noninterest expense				36
Provision for Loan and Lease Losses				(8)
Income before income taxes and including noncontrolling interests			113	107
Income tax expense (benefit)				42
Net Income Including Noncontrolling Interests			68	65
Net Income Attributable to MUAH			68	65
Total assets, end of period		6,677		6,677
US Corporate Banking | Operating segments
Business segments
Net interest income (expense)	55	48	108
Noninterest income (expense)	48	24	83
Total revenue	103	72
Noninterest expense	45	18	92
Provision for credit losses	(11)	8	(14)
Income before income taxes and including noncontrolling interests	69	46
Income tax expense (benefit)	27	18	45
Net Income Including Noncontrolling Interests	42	28
Net Income Attributable to MUAH	42	28
Total assets, end of period	7,386	6,677	7,386	6,677
Transaction Banking | Operating segments
Business segments
Net interest income (expense)	111	109	226	214
Noninterest income (expense)	45	39	92	79
Total revenue	156	148	318	293
Noninterest expense	103	88	205	176
Provision for Loan and Lease Losses				3
Provision for credit losses	0	1	1
Income before income taxes and including noncontrolling interests	53	59	112	114
Income tax expense (benefit)	21	24	44	45
Net Income Including Noncontrolling Interests	32	35	68	69
Net Income Attributable to MUAH	32	35	68	69
Total assets, end of period	1,778	1,876	1,778	1,876
Investment Banking & Markets | Operating segments
Business segments
Net interest income (expense)	37	43	71	89
Noninterest income (expense)	114	57	177	107
Total revenue	151	100	248	196
Noninterest expense	63	27	115	55
Provision for Loan and Lease Losses				30
Provision for credit losses	(9)	15	(23)
Income before income taxes and including noncontrolling interests	97	58	156	111
Income tax expense (benefit)	26	11	35	23
Net Income Including Noncontrolling Interests	71	47	121	88
Net Income Attributable to MUAH	71	47	121	88
Total assets, end of period	6,222	6,199	6,222	6,199
Other | Operating segments
Business segments
Net interest income (expense)	58	21	78	28
Noninterest income (expense)	100	(7)	209	(12)
Total revenue	158	14	287	16
Noninterest expense	225	114	473	221
Provision for Loan and Lease Losses				2
Provision for credit losses	7	1	8
Income before income taxes and including noncontrolling interests	(74)	(101)	(194)	(207)
Income tax expense (benefit)	(26)	(57)	(72)	(98)
Net Income Including Noncontrolling Interests	(48)	(44)	(122)	(109)
Deduct: net loss from noncontrolling interests	6	4	11	9
Net Income Attributable to MUAH	(42)	(40)	(111)	(100)
Total assets, end of period	$ 28,516	$ 27,557	$ 28,516	$ 27,557