Loans and Allowance for Loan Losses - Pre and Post Modification Outstanding Recorded Investment Amounts of TDRs (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	$ 362	$ 353
Post-Modification Outstanding Recorded Investment	341	352
Commercial
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	343	337
Post-Modification Outstanding Recorded Investment	322	336
Commercial \| Commercial and industrial
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	186	327
Post-Modification Outstanding Recorded Investment	186	326
Commercial \| Commercial mortgage
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	5	10
Post-Modification Outstanding Recorded Investment	5	10
Commercial \| Construction
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	152	0
Post-Modification Outstanding Recorded Investment	131	0
Consumer
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	19	16
Post-Modification Outstanding Recorded Investment	19	16
Consumer \| Residential mortgage
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	16	11
Post-Modification Outstanding Recorded Investment	16	11
Consumer \| Home equity and other consumer loans
Pre- and Post - modification recorded investments
Pre-Modification Outstanding Recorded Investment	3	5
Post-Modification Outstanding Recorded Investment	$ 3	$ 5

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ub_CommercialLoansPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ProductOrServiceAxis=ub_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ProductOrServiceAxis=ub_CommercialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ProductOrServiceAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ub_ConsumerLoansPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ProductOrServiceAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ProductOrServiceAxis=ub_HomeEquityAndOtherConsumerLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: