Business Segments (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Business segments
Net interest income (expense)	$ 763	$ 672	$ 1,446	$ 1,325
Noninterest income (expense)	202	201	383	452
Total revenue	965	873	1,829	1,777
Noninterest expense	649	702	1,309	1,415
(Reversal of) provision for loan losses	9	(3)	(7)	(6)
Income (loss) before income taxes and including noncontrolling interests	307	174	527	368
Income tax expense (benefit)	62	35	112	85
Net income (loss) including noncontrolling interests	245	139	415	283
Deduct: net loss from noncontrolling interests	4	3	9	7
Net Income Attributable to MUFG Americas Holdings Corporation (MUAH)	249	142	424	290
Total assets, end of period	108,820	102,279	108,820	102,279	105,894
Reconciling Items
Business segments
Net interest income (expense)	(92)	(76)	(172)	(153)
Noninterest income (expense)	(55)	(51)	(102)	(92)
Total revenue	(147)	(127)	(274)	(245)
Noninterest expense	(38)	(32)	(73)	(65)
(Reversal of) provision for loan losses	(8)	(1)	(19)
Income (loss) before income taxes and including noncontrolling interests	(101)	(94)	(182)	(180)
Income tax expense (benefit)	(39)	(36)	(71)	(71)
Net income (loss) including noncontrolling interests	(62)	(58)	(111)	(109)
Net Income Attributable to MUFG Americas Holdings Corporation (MUAH)	(62)	(58)	(111)	(109)
Total assets, end of period	(3,558)	(1,863)	(3,558)	(1,863)
Retail Banking and Wealth Markets | Operating segments
Business segments
Net interest income (expense)	362	332	709	669
Noninterest income (expense)	87	94	166	187
Total revenue	449	426	875	856
Noninterest expense	340	366	675	730
(Reversal of) provision for loan losses	(2)	(7)	(8)	(15)
Income (loss) before income taxes and including noncontrolling interests	111	67	208	141
Income tax expense (benefit)	44	27	82	56
Net income (loss) including noncontrolling interests	67	40	126	85
Net Income Attributable to MUFG Americas Holdings Corporation (MUAH)	67	40	126	85
Total assets, end of period	36,417	33,008	36,417	33,008
Commercial Banking | Operating segments
Business segments
Net interest income (expense)	277	217	504	423
Noninterest income (expense)	58	49	106	92
Total revenue	335	266	610	515
Noninterest expense	104	102	209	199
(Reversal of) provision for loan losses	(10)	3	6	30
Income (loss) before income taxes and including noncontrolling interests	241	161	395	286
Income tax expense (benefit)	65	40	99	69
Net income (loss) including noncontrolling interests	176	121	296	217
Net Income Attributable to MUFG Americas Holdings Corporation (MUAH)	176	121	296	217
Total assets, end of period	35,423	31,153	35,423	31,153
Corporate Banking | Operating segments
Business segments
Net interest income (expense)	43	31	74	60
Noninterest income (expense)	22	18	40	35
Total revenue	65	49	114	95
Noninterest expense	16	12	31	26
(Reversal of) provision for loan losses	9	(4)	(7)	2
Income (loss) before income taxes and including noncontrolling interests	40	41	90	67
Income tax expense (benefit)	16	15	35	26
Net income (loss) including noncontrolling interests	24	26	55	41
Net Income Attributable to MUFG Americas Holdings Corporation (MUAH)	24	26	55	41
Total assets, end of period	4,916	4,364	4,916	4,364
Transaction Banking | Operating segments
Business segments
Net interest income (expense)	94	95	194	206
Noninterest income (expense)	39	41	79	79
Total revenue	133	136	273	285
Noninterest expense	88	91	177	184
(Reversal of) provision for loan losses	2	(4)	3	(3)
Income (loss) before income taxes and including noncontrolling interests	43	49	93	104
Income tax expense (benefit)	17	20	36	41
Net income (loss) including noncontrolling interests	26	29	57	63
Net Income Attributable to MUFG Americas Holdings Corporation (MUAH)	26	29	57	63
Total assets, end of period	1,938	1,507	1,938	1,507
Investment Banking & Markets | Operating segments
Business segments
Net interest income (expense)	43	43	89	89
Noninterest income (expense)	57	54	107	101
Total revenue	100	97	196	190
Noninterest expense	28	25	55	50
(Reversal of) provision for loan losses	15	7	30	4
Income (loss) before income taxes and including noncontrolling interests	57	65	111	136
Income tax expense (benefit)	12	17	23	36
Net income (loss) including noncontrolling interests	45	48	88	100
Net Income Attributable to MUFG Americas Holdings Corporation (MUAH)	45	48	88	100
Total assets, end of period	6,199	5,585	6,199	5,585
Other | Operating segments
Business segments
Net interest income (expense)	36	30	48	31
Noninterest income (expense)	(6)	(4)	(13)	50
Total revenue	30	26	35	81
Noninterest expense	111	138	235	291
(Reversal of) provision for loan losses	3	3	(12)	(24)
Income (loss) before income taxes and including noncontrolling interests	(84)	(115)	(188)	(186)
Income tax expense (benefit)	(53)	(48)	(92)	(72)
Net income (loss) including noncontrolling interests	(31)	(67)	(96)	(114)
Deduct: net loss from noncontrolling interests	4	3	9	7
Net Income Attributable to MUFG Americas Holdings Corporation (MUAH)	(27)	(64)	(87)	(107)
Total assets, end of period	$ 27,485	$ 28,525	$ 27,485	$ 28,525