Business Segments (Details 2) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Business segments
Net interest income (expense)	$ 705	$ 707	$ 2,107	$ 2,153
Noninterest income (expense)	397	388	1,117	771
Total revenue	1,102	1,095	3,224	2,924
Noninterest expense	855	764	2,547	2,026
Provision for Loan and Lease Losses	23	(18)	46	(25)
Provision for credit losses	18	1	36	7
Income before income taxes and including noncontrolling interests	229	330	641	891
Income tax expense (benefit)	64	88	169	242
Net Income Including Noncontrolling Interests	165	242	472	649
Deduct: net loss from noncontrolling interests	21	5	32	14
Net Income Attributable to MUAH	186	247	504	663
Total assets, end of period	115,157	110,867	115,157	110,867	$ 113,623
Reconciling Items
Business segments
Net interest income (expense)	(99)	(102)	(295)	(290)
Noninterest income (expense)	(50)	(64)	(151)	(166)
Total revenue	(149)	(166)	(446)	(456)
Noninterest expense	(46)	(41)	(138)	(114)
Provision for credit losses	0	9	5	(10)
Income before income taxes and including noncontrolling interests	(103)	(134)	(313)	(332)
Income tax expense (benefit)	(40)	(52)	(123)	(130)
Net Income Including Noncontrolling Interests	(63)	(82)	(190)	(202)
Net Income Attributable to MUAH	(63)	(82)	(190)	(202)
Total assets, end of period	(4,191)	(3,762)	(4,191)	(3,762)
Retail Banking and Wealth Markets | Operating segments
Business segments
Net interest income (expense)	332	340	977	1,049
Noninterest income (expense)	96	83	265	249
Total revenue	428	423	1,242	1,298
Noninterest expense	348	329	1,063	1,005
Provision for credit losses	1	(1)	9	(9)
Income before income taxes and including noncontrolling interests	79	95	170	302
Income tax expense (benefit)	31	38	66	119
Net Income Including Noncontrolling Interests	48	57	104	183
Net Income Attributable to MUAH	48	57	104	183
Total assets, end of period	36,480	37,235	36,480	37,235
Commercial Banking | Operating segments
Business segments
Net interest income (expense)	235	242	706	746
Noninterest income (expense)	46	55	136	155
Total revenue	281	297	842	901
Noninterest expense	97	92	282	262
Provision for credit losses	8	5	41	11
Income before income taxes and including noncontrolling interests	176	200	519	628
Income tax expense (benefit)	51	62	152	201
Net Income Including Noncontrolling Interests	125	138	367	427
Net Income Attributable to MUAH	125	138	367	427
Total assets, end of period	37,336	35,296	37,336	35,296
Corporate Banking | Operating segments
Business segments
Net interest income (expense)				143
Noninterest income (expense)				92
Total revenue				235
Noninterest expense				77
Provision for credit losses				(12)
Income before income taxes and including noncontrolling interests				170
Income tax expense (benefit)				67
Net Income Including Noncontrolling Interests			94	103
Net Income Attributable to MUAH			94	103
Total assets, end of period		6,965		6,965
US Corporate Banking | Operating segments
Business segments
Net interest income (expense)	54	53	162
Noninterest income (expense)	39	46	121
Total revenue	93	99	283
Noninterest expense	46	42	138
Provision for credit losses	4	(4)	(10)
Income before income taxes and including noncontrolling interests	43	61	155
Income tax expense (benefit)	17	24	61
Net Income Including Noncontrolling Interests	26	37
Net Income Attributable to MUAH	26	37
Total assets, end of period	8,187	6,965	8,187	6,965
Transaction Banking | Operating segments
Business segments
Net interest income (expense)	114	110	340	324
Noninterest income (expense)	47	44	139	123
Total revenue	161	154	479	447
Noninterest expense	108	91	312	266
Provision for credit losses	0	(6)	1	(3)
Income before income taxes and including noncontrolling interests	53	69	166	184
Income tax expense (benefit)	20	27	65	72
Net Income Including Noncontrolling Interests	33	42	101	112
Net Income Attributable to MUAH	33	42	101	112
Total assets, end of period	1,835	1,753	1,835	1,753
Investment Banking & Markets | Operating segments
Business segments
Net interest income (expense)	33	45	104	134
Noninterest income (expense)	70	138	247	245
Total revenue	103	183	351	379
Noninterest expense	58	54	173	109
Provision for credit losses	(1)	(8)	(24)	23
Income before income taxes and including noncontrolling interests	46	137	202	247
Income tax expense (benefit)	7	42	41	63
Net Income Including Noncontrolling Interests	39	95	161	184
Net Income Attributable to MUAH	39	95	161	184
Total assets, end of period	6,213	6,160	6,213	6,160
Other | Operating segments
Business segments
Net interest income (expense)	36	19	113	47
Noninterest income (expense)	149	86	360	73
Total revenue	185	105	473	120
Noninterest expense	244	197	717	421
Provision for credit losses	6	6	14	7
Income before income taxes and including noncontrolling interests	(65)	(98)	(258)	(308)
Income tax expense (benefit)	(22)	(53)	(93)	(150)
Net Income Including Noncontrolling Interests	(43)	(45)	(165)	(158)
Deduct: net loss from noncontrolling interests	21	5	32	14
Net Income Attributable to MUAH	(22)	(40)	(133)	(144)
Total assets, end of period	$ 29,297	$ 27,220	$ 29,297	$ 27,220