Name:	ub_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
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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_
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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
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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
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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
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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
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This item represents the fair value of securities categorized as held-to-maturity which are in an unrealized loss position.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

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

This item represents the fair value of securities categorized as held-to-maturity that have been in a continuous unrealized loss position for less than twelve months.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

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

This item represents the fair value of securities categorized as held-to-maturity securities that have been in a continuous unrealized loss position for twelve months or longer.