Commitments, Contingencies and Guarantees (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($)
Financial guarantee \| Interest rate swap contracts
Commitment, Contingencies and Guarantees disclosures
Current exposure to loss	$ 26
Future exposure to loss	43
Commitments to extend credit
Commitment, Contingencies and Guarantees disclosures
Commitments	38,442
Issued standby and commercial letters of credit
Commitment, Contingencies and Guarantees disclosures
Commitments	$ 4,191
Maximum term	1 year
Loss Contingency Accrual	$ 4
Commitments to enter into forward-starting resale agreements
Commitment, Contingencies and Guarantees disclosures
Commitments	1,150
Other commitments
Commitment, Contingencies and Guarantees disclosures
Commitments	556
MUSA and Various other Non-Bank Subsidiaries \| Unsecured Debt [Member]
Commitment, Contingencies and Guarantees disclosures
Revolving credit facility borrowing capacity	1,500
Commercial \| Notes Receivable [Member] \| Financial and Insurance [Member]
Commitment, Contingencies and Guarantees disclosures
Commitments	9,000
Commercial \| Notes Receivable [Member] \| Power and Utilities [Member]
Commitment, Contingencies and Guarantees disclosures
Commitments	5,000
Commercial \| Notes Receivable [Member] \| Oil and Gas [Member]
Commitment, Contingencies and Guarantees disclosures
Commitments	2,000
Commercial \| Notes Receivable [Member] \| Manufacturing Sector [Member]
Commitment, Contingencies and Guarantees disclosures
Commitments	3,000
Commercial \| Notes Receivable [Member] \| Commercial mortgage
Commitment, Contingencies and Guarantees disclosures
Commitments	$ 5,000

Represents the maximum term of the commitment.

No definition available.

Name:	ub_OtherCommitmentTermMaximum
Namespace Prefix:	ub_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118943654&loc=d3e12069-110248

+ Details

Name:	us-gaap_GuaranteeObligationsCurrentCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=118943654&loc=d3e12069-110248

+ Details

Name:	us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of loss contingency liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942415&loc=d3e14326-108349

+ Details

Name:	us-gaap_LossContingencyAccrualAtCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherCommitment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

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