Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Jan. 31, 2019
Document And Entity Information
Entity Registrant Name	MUFG Americas Holdings Corp
Entity Central Index Key	0001011659
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Public Float	$ 0
Entity Common Stock, Shares Outstanding		131,935,124
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY