Business Segments - Summary of Results (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Business segments
Net interest income (expense)	$ 783	$ 825
Noninterest income (expense)	632	382
Total revenue	1,415	1,207
Noninterest expense	1,170	1,084
(Reversal of) provision for credit losses	38	(2)
Income before income taxes and including noncontrolling interests	207	125
Income tax expense (benefit)	28	(42)
Net income including noncontrolling interests	179	167
Deduct: net loss from noncontrolling interests	5	(1)
Net Income Attributable to MUAH	184	166
Total assets, end of period	170,707	157,310	$ 168,100
Operating segments | Regional Bank
Business segments
Net interest income (expense)	571	526
Noninterest income (expense)	129	111
Total revenue	700	637
Noninterest expense	539	502
(Reversal of) provision for credit losses	47	12
Income before income taxes and including noncontrolling interests	114	123
Income tax expense (benefit)	25	25
Net income including noncontrolling interests	89	98
Deduct: net loss from noncontrolling interests	0	0
Net Income Attributable to MUAH	89	98
Total assets, end of period	74,396	68,940
Operating segments | U.S. Wholesale & Investment Banking
Business segments
Net interest income (expense)	100	102
Noninterest income (expense)	88	81
Total revenue	188	183
Noninterest expense	111	102
(Reversal of) provision for credit losses	(6)	(18)
Income before income taxes and including noncontrolling interests	83	99
Income tax expense (benefit)	13	138
Net income including noncontrolling interests	70	(39)
Deduct: net loss from noncontrolling interests	0	0
Net Income Attributable to MUAH	70	(39)
Total assets, end of period	20,719	20,032
Operating segments | Transaction Banking
Business segments
Net interest income (expense)	68	64
Noninterest income (expense)	15	14
Total revenue	83	78
Noninterest expense	73	57
(Reversal of) provision for credit losses	(1)	(1)
Income before income taxes and including noncontrolling interests	11	22
Income tax expense (benefit)	3	6
Net income including noncontrolling interests	8	16
Deduct: net loss from noncontrolling interests	0	0
Net Income Attributable to MUAH	8	16
Total assets, end of period	797	933
Operating segments | MUSA
Business segments
Net interest income (expense)	35	54
Noninterest income (expense)	93	88
Total revenue	128	142
Noninterest expense	123	116
(Reversal of) provision for credit losses	0	0
Income before income taxes and including noncontrolling interests	5	26
Income tax expense (benefit)	0	6
Net income including noncontrolling interests	5	20
Deduct: net loss from noncontrolling interests	0	0
Net Income Attributable to MUAH	5	20
Total assets, end of period	33,930	32,661
Segment Reconciling Items
Business segments
Net interest income (expense)	9	79
Noninterest income (expense)	307	88
Total revenue	316	167
Noninterest expense	324	307
(Reversal of) provision for credit losses	(2)	5
Income before income taxes and including noncontrolling interests	(6)	(145)
Income tax expense (benefit)	(13)	(217)
Net income including noncontrolling interests	7	72
Deduct: net loss from noncontrolling interests	5	(1)
Net Income Attributable to MUAH	12	71
Total assets, end of period	$ 40,865	$ 34,744