Name:	ub_HeldToMaturitySecuritiesAmortizedCost
Namespace Prefix:	ub_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Held to maturity securities continuous unrealized loss position less than twelve months aggregate losses not recognized in other comprehensive income.

No definition available.

Name:	ub_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLossesNotRecognizedInOtherComprehensiveIncome
Namespace Prefix:	ub_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Held to maturity securities continuous unrealized loss position less than twelve months aggregate losses recognized in other comprehensive income.

No definition available.

Name:	ub_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLossesRecognizedInOtherComprehensiveIncome
Namespace Prefix:	ub_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Held to maturity securities continuous unrealized loss position twelve months or longer aggregate losses not recognized in other comprehensive income.

No definition available.

Name:	ub_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerAggregateLossesNotRecognizedInOtherComprehensiveIncome
Namespace Prefix:	ub_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Held to maturity securities continuous unrealized loss position twelve months or longer aggregate losses recognized in other comprehensive income.

No definition available.

Name:	ub_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerAggregateLossesRecognizedInOtherComprehensiveIncome
Namespace Prefix:	ub_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Unrealized holding gain on securities transferred to held-to-maturity securities classification as of the transfer date, net of amortization and other-than-temporary impairments recognized subsequent to the date of transfer.

No definition available.

Name:	ub_HeldToMaturitySecuritiesUnrealizedHoldingGainRecognizedInOtherComprehensiveIncome
Namespace Prefix:	ub_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Unrealized holding loss on securities transferred to held-to-maturity securities classification as of the transfer date, net of amortization and other-than-temporary impairments recognized subsequent to the date of transfer.

No definition available.

Name:	ub_HeldToMaturitySecuritiesUnrealizedHoldingLossesRecognizedInOtherComprehensiveIncome
Namespace Prefix:	ub_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of investment in debt security measured at amortized cost (held-to-maturity).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

Name:	us-gaap_HeldToMaturitySecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

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

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

Name:	us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27290-111563

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for less than 12 months.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27337-111563

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for 12 months or longer.

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Fair value of investment in debt security measured at amortized cost (held-to-maturity).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

Name:	us-gaap_HeldToMaturitySecuritiesFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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.