Cover - shares	6 Months Ended
	Jun. 30, 2021	Jul. 31, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2021
Document Transition Report	false
Entity File Number	1-15081
Entity Registrant Name	MUFG Americas Holdings Corporation
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	94-1234979
Entity Address, Address Line One	1251 Avenue of the Americas
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10020
City Area Code	(212)
Local Phone Number	782-6800
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		132,076,912
Entity Central Index Key	0001011659
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2