Investments (Invested Assets on Deposit, Held In Trust and Pledged as Collateral) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Invested assets on deposit (regulatory deposits)	$ 7,443us-gaap_DepositAssets	$ 7,334us-gaap_DepositAssets
Invested assets held-in-trust (reinsurance agreements)	957us-gaap_AssetsHeldInTrust	936us-gaap_AssetsHeldInTrust
Invested assets pledged as collateral	3,591mlus_InvestedAssetsPledgedAsCollateral	3,174mlus_InvestedAssetsPledgedAsCollateral
Total invested assets on deposit, held in trust and pledged as collateral	$ 11,991	$ 11,444

The sum of the fair values, as of the date of each statement of financial position presented, of securities that are restricted as collateral for the payment of related debt obligations, primarily secured borrowings and repurchase agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

No definition available.

+ Details

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Namespace Prefix:	mlus_
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Balance Type:	debit
Period Type:	instant

- Definition

Invested assets pledged as collateral.

+ References

No definition available.

+ Details

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- Definition

The total amount of cash and securities held by third party trustees pursuant to terms of debt instruments or other agreements as of the date of each statement of financial position presented, which can be used by the trustee only to pay the noncurrent portion of specified obligations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(b))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

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- Definition

The carrying amount of the asset transferred to a third party to serve as a deposit, which typically serves as security against failure by the transferor to perform under terms of an agreement.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 17

-Article 5

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 8

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.8,17)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Balance Type:	debit
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- Details

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