Business Segments - Schedule of Segments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business segments
Number of Reportable Segments	4
Net interest income (expense)	$ 3,307	$ 3,204	$ 3,053
Noninterest income	2,177	2,010	2,225
Total income	5,484	5,214	5,278
Other expense	4,277	3,984	3,782
(Reversal of) provision for credit losses	106	(103)	155
Income before income taxes and including noncontrolling interests	1,101	1,333	1,341
Income tax benefit	52	299	419
Net Income	1,049	1,034	922
Deduct: Net loss from noncontrolling interests	24	43	68
Net Income Attributable to MUAH	1,073	1,077	990
Total assets, end of period	168,100	154,550	148,144
Other
Business segments
Net interest income (expense)	233	255	194
Noninterest income	994	783	1,034
Total income	1,227	1,038	1,228
Other expense	1,103	946	983
(Reversal of) provision for credit losses	2	(22)	27
Income before income taxes and including noncontrolling interests	122	114	218
Income tax benefit	(211)	226	96
Net Income	333	(112)	122
Deduct: Net loss from noncontrolling interests	24	43	68
Net Income Attributable to MUAH	357	(69)	190
Total assets, end of period	38,828	33,160	30,713
Regional Bank | Operating segments
Business segments
Net interest income (expense)	2,195	2,051	1,952
Noninterest income	451	442	449
Total income	2,646	2,493	2,401
Other expense	2,058	1,961	1,843
(Reversal of) provision for credit losses	65	22	30
Income before income taxes and including noncontrolling interests	523	510	528
Income tax benefit	105	178	152
Net Income	418	332	376
Deduct: Net loss from noncontrolling interests	0	0	0
Net Income Attributable to MUAH	418	332	376
Total assets, end of period	73,554	67,804	63,169
U.S. Wholesale Banking and Investment Banking [Member] | Operating segments
Business segments
Net interest income (expense)	415
Noninterest income	373
Other expense	418
(Reversal of) provision for credit losses	42
Income tax benefit	122
Deduct: Net loss from noncontrolling interests	0
Total assets, end of period	20,933
U.S. Wholesale & Investment Banking | Operating segments
Business segments
Net interest income (expense)		417	541
Noninterest income		370	368
Total income	788	787	909
Other expense		403	404
(Reversal of) provision for credit losses		(104)	98
Income before income taxes and including noncontrolling interests	328	488	407
Income tax benefit		(197)	91
Net Income	206	685	316
Deduct: Net loss from noncontrolling interests		0	0
Net Income Attributable to MUAH	206	685	316
Total assets, end of period		20,461	23,805
Transaction Banking | Operating segments
Business segments
Net interest income (expense)	264	247	202
Noninterest income	59	64	72
Total income	323	311	274
Other expense	255	229	190
(Reversal of) provision for credit losses	(3)	1	0
Income before income taxes and including noncontrolling interests	71	81	84
Income tax benefit	19	33	39
Net Income	52	48	45
Deduct: Net loss from noncontrolling interests	0	0	0
Net Income Attributable to MUAH	52	48	45
Total assets, end of period	941	1,063	1,205
MUSA | Operating segments
Business segments
Net interest income (expense)	200	234	164
Noninterest income	300	351	302
Total income	500	585	466
Other expense	443	445	362
(Reversal of) provision for credit losses	0	0	0
Income before income taxes and including noncontrolling interests	57	140	104
Income tax benefit	17	59	41
Net Income	40	81	63
Deduct: Net loss from noncontrolling interests	0	0	0
Net Income Attributable to MUAH	40	81	63
Total assets, end of period	$ 33,844	$ 32,062	$ 29,252