Loans and Allowance for Loan Losses (Details 14) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Loan impairment
Average Recorded Investment	$ 752	$ 448	$ 765	$ 386
Recognized Interest Income	4	2	10	7
Commercial portfolio
Loan impairment
Average Recorded Investment	491	154	501	88
Recognized Interest Income	1	0	5	2
Commercial portfolio \| Commercial and industrial
Loan impairment
Average Recorded Investment	419	139	447	74
Recognized Interest Income	0	0	4	2
Commercial portfolio \| Commercial mortgage
Loan impairment
Average Recorded Investment	61	15	40	14
Recognized Interest Income	1	0	1	0
Commercial portfolio \| Construction
Loan impairment
Average Recorded Investment	11	0	14	0
Recognized Interest Income	0	0	0	0
Consumer portfolio
Loan impairment
Average Recorded Investment	261	294	264	298
Recognized Interest Income	3	2	5	5
Consumer portfolio \| Residential mortgage
Loan impairment
Average Recorded Investment	233	263	235	267
Recognized Interest Income	2	2	4	4
Consumer portfolio \| Home equity and other consumer loans
Loan impairment
Average Recorded Investment	28	31	29	31
Recognized Interest Income	$ 1	$ 0	$ 1	$ 1

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

- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5258-111524

+ Details

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

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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: