Business Segments - Schedule of Segments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business segments
Number of Reportable Segments	4
Net interest income (expense)	$ 3,093	$ 3,307	$ 3,204
Noninterest income	2,705	2,177	2,010
Total income	5,798	5,484	5,214
Other expense	6,215	4,277	3,984
(Reversal of) provision for credit losses	252	106	(103)
Income before income taxes and including noncontrolling interests	(669)	1,101	1,333
Income tax benefit	82	52	299
Net Income	(751)	1,049	1,034
Deduct: Net loss (income) from noncontrolling interests	17	24	43
Net (Loss) Income Attributable to MUAH	(734)	1,073	1,077
Total assets, end of period	170,810	168,100	154,550
Other (1)
Business segments
Net interest income (expense)	(51)	233	255
Noninterest income	1,392	994	783
Total income	1,341	1,227	1,038
Other expense	2,907	1,103	946
(Reversal of) provision for credit losses	(6)	2	(22)
Income before income taxes and including noncontrolling interests	(1,560)	122	114
Income tax benefit	(66)	(211)	226
Net Income	(1,494)	333	(112)
Deduct: Net loss (income) from noncontrolling interests	17	24	43
Net (Loss) Income Attributable to MUAH	(1,477)	357	(69)
Total assets, end of period	39,548	38,828	33,160
Regional Bank | Operating segments
Business segments
Net interest income (expense)	2,294	2,195	2,051
Noninterest income	520	451	442
Total income	2,814	2,646	2,493
Other expense	2,130	2,058	1,961
(Reversal of) provision for credit losses	209	65	22
Income before income taxes and including noncontrolling interests	475	523	510
Income tax benefit	93	105	178
Net Income	382	418	332
Deduct: Net loss (income) from noncontrolling interests	0	0	0
Net (Loss) Income Attributable to MUAH	382	418	332
Total assets, end of period	74,980	73,554	67,804
U.S. Wholesale Banking and Investment Banking [Member] | Operating segments
Business segments
Net interest income (expense)	424
Noninterest income	353
Other expense	458
(Reversal of) provision for credit losses	51
Income tax benefit	12
Deduct: Net loss (income) from noncontrolling interests	0
Total assets, end of period	20,822
Global Corporate & Investment Banking - U.S. | Operating segments
Business segments
Net interest income (expense)		415	417
Noninterest income		373	370
Total income	777	788	787
Other expense		418	403
(Reversal of) provision for credit losses		42	(104)
Income before income taxes and including noncontrolling interests	268	328	488
Income tax benefit		122	(197)
Net Income	256	206	685
Deduct: Net loss (income) from noncontrolling interests		0	0
Net (Loss) Income Attributable to MUAH	256	206	685
Total assets, end of period		20,933	20,461
Transaction Banking | Operating segments
Business segments
Net interest income (expense)	259	264	247
Noninterest income	56	59	64
Total income	315	323	311
Other expense	244	255	229
(Reversal of) provision for credit losses	(2)	(3)	1
Income before income taxes and including noncontrolling interests	73	71	81
Income tax benefit	25	19	33
Net Income	48	52	48
Deduct: Net loss (income) from noncontrolling interests	0	0	0
Net (Loss) Income Attributable to MUAH	48	52	48
Total assets, end of period	1,015	941	1,063
MUSA | Operating segments
Business segments
Net interest income (expense)	167	200	234
Noninterest income	384	300	351
Total income	551	500	585
Other expense	476	443	445
(Reversal of) provision for credit losses	0	0	0
Income before income taxes and including noncontrolling interests	75	57	140
Income tax benefit	18	17	59
Net Income	57	40	81
Deduct: Net loss (income) from noncontrolling interests	0	0	0
Net (Loss) Income Attributable to MUAH	57	40	81
Total assets, end of period	$ 34,445	$ 33,844	$ 32,062