Business Segments - Summary of Results (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Business segments
Net interest income (expense)	$ 774	$ 783
Noninterest income (expense)	612	632
Total revenue	1,386	1,415
Noninterest expense	1,201	1,170
(Reversal of) provision for credit losses	470	38
Income before income taxes and including noncontrolling interests	(285)	207
Income tax expense (benefit)	25	28
Net (loss) income including noncontrolling interests	(310)	179
Deduct: net loss from noncontrolling interests	4	5
Net Income Attributable to MUAH	(306)	184
Total assets, end of period	165,696	170,707	$ 170,810
Operating segments | Regional Bank
Business segments
Net interest income (expense)	545	571
Noninterest income (expense)	128	129
Total revenue	673	700
Noninterest expense	530	539
(Reversal of) provision for credit losses	367	47
Income before income taxes and including noncontrolling interests	(224)	114
Income tax expense (benefit)	(41)	25
Net (loss) income including noncontrolling interests	(183)	89
Deduct: net loss from noncontrolling interests	0	0
Net Income Attributable to MUAH	(183)	89
Total assets, end of period	74,068	74,396
Operating segments | Global Corporate & Investment Banking - U.S.
Business segments
Net interest income (expense)	112	100
Noninterest income (expense)	86	88
Total revenue	198	188
Noninterest expense	120	111
(Reversal of) provision for credit losses	110	(6)
Income before income taxes and including noncontrolling interests	(32)	83
Income tax expense (benefit)	(23)	13
Net (loss) income including noncontrolling interests	(9)	70
Deduct: net loss from noncontrolling interests	0	0
Net Income Attributable to MUAH	(9)	70
Total assets, end of period	23,037	20,719
Operating segments | Transaction Banking
Business segments
Net interest income (expense)	60	68
Noninterest income (expense)	14	15
Total revenue	74	83
Noninterest expense	55	73
(Reversal of) provision for credit losses	0	(1)
Income before income taxes and including noncontrolling interests	19	11
Income tax expense (benefit)	4	3
Net (loss) income including noncontrolling interests	15	8
Deduct: net loss from noncontrolling interests	0	0
Net Income Attributable to MUAH	15	8
Total assets, end of period	1,399	797
Operating segments | MUSA
Business segments
Net interest income (expense)	66	35
Noninterest income (expense)	51	93
Total revenue	117	128
Noninterest expense	116	123
(Reversal of) provision for credit losses	0	0
Income before income taxes and including noncontrolling interests	1	5
Income tax expense (benefit)		0
Net (loss) income including noncontrolling interests	1	5
Deduct: net loss from noncontrolling interests	0	0
Net Income Attributable to MUAH	1	5
Total assets, end of period	27,534	33,930
Segment Reconciling Items
Business segments
Net interest income (expense)	(9)	9
Noninterest income (expense)	333	307
Total revenue	324	316
Noninterest expense	380	324
(Reversal of) provision for credit losses	(7)	(2)
Income before income taxes and including noncontrolling interests	(49)	(6)
Income tax expense (benefit)	85	(13)
Net (loss) income including noncontrolling interests	(134)	7
Deduct: net loss from noncontrolling interests	4	5
Net Income Attributable to MUAH	(130)	12
Total assets, end of period	$ 39,658	$ 40,865