Securities - Carrying Amount and Fair Value of Securities Held to Maturity by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Securities held to maturity
Over One Year Through Five Years, Carrying Amount	$ 1,407
Over One Year Through Five Years, Fair Value	1,446
Over Five Years Through Ten Years, Carrying Amount	460
Over Five Years Through Ten Years, Fair Value	461
Over Ten Years, Carrying Amount	8,476
Over Ten Years, Fair Value	8,442
Total securities held to maturity, carrying amount	10,343	$ 10,337
Securities held to maturity	10,349	10,316
U.S. Treasury
Securities held to maturity
Over One Year Through Five Years, Carrying Amount	524
Over One Year Through Five Years, Fair Value	530
Over Five Years Through Ten Years, Carrying Amount	0
Over Five Years Through Ten Years, Fair Value	0
Over Ten Years, Carrying Amount	0
Over Ten Years, Fair Value	0
Total securities held to maturity, carrying amount	524	492
Securities held to maturity	530	497
U.S. government agency and government-sponsored agencies - residential mortgage-backed securities
Securities held to maturity
Over One Year Through Five Years, Carrying Amount	100
Over One Year Through Five Years, Fair Value	100
Over Five Years Through Ten Years, Carrying Amount	391
Over Five Years Through Ten Years, Fair Value	389
Over Ten Years, Carrying Amount	7,801
Over Ten Years, Fair Value	7,763
Total securities held to maturity, carrying amount	8,292	8,263
Securities held to maturity	8,252	$ 8,201
U.S. government agency and government-sponsored agencies - commercial mortgage-backed securities
Securities held to maturity
Over One Year Through Five Years, Carrying Amount	783
Over One Year Through Five Years, Fair Value	816
Over Five Years Through Ten Years, Carrying Amount	69
Over Five Years Through Ten Years, Fair Value	72
Over Ten Years, Carrying Amount	675
Over Ten Years, Fair Value	679
Total securities held to maturity, carrying amount	1,527
Securities held to maturity	$ 1,567

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.6)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Fair value of held-to-maturity debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- Definition

Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized, and fair value hedge accounting adjustments, maturing in the fifth through tenth fiscal year following the latest fiscal year.

+ References

+ Details

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

- Definition

Fair value of held-to-maturity debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62586-112803

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)(2)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

+ Details

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

- Definition

Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized, and fair value hedge accounting adjustments, maturing in the next fiscal year through the fifth fiscal year following the latest fiscal year.

+ References

+ Details

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

- Definition

Fair value of held-to-maturity debt securities maturing after the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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

- Definition

Carrying amount of debt securities accounted for as held-to-maturity, net of adjustments including, but not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized, and fair value hedge accounting adjustments, maturing after the tenth fiscal year following the latest fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62586-112803

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)(4)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

+ Details

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

- Definition

Fair value of investments in debt securities classified as held-to-maturity.

+ References

+ Details

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

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

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=ub_ResidentialMortgageBackedSecuritiesUsGovernmentAgenciesAndGovernmentSponsoredAgenciesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=ub_CommercialMortgageBackedSecuritiesUSGovernmentAgenciesAndGovernmentSponsoredAgenciesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: