Fair Value (Financial Instruments Carried at Other Than Fair Value) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Assets
Policy loans	$ 8,491us-gaap_LoansInsurancePolicy	$ 8,421us-gaap_LoansInsurancePolicy
Liabilities
Separate account liabilities	139,335us-gaap_SeparateAccountsLiability	134,796us-gaap_SeparateAccountsLiability
Estimated Fair Value [Member]
Assets
Mortgage loans	50,992	47,369
Policy loans	10,410	9,553
Real estate joint ventures	54	70
Other limited partnership interests	819	1,013
Other invested assets	2,490	2,015
Premiums, reinsurance and other receivables	14,701	14,921
Liabilities
PABs	75,481	72,236
Long-term debt	2,297	2,956
Other liabilities	20,742	20,097
Separate account liabilities	60,840	57,935
Estimated Fair Value [Member] \| Level 1
Assets
Mortgage loans	0	0
Policy loans	0	0
Real estate joint ventures	0	0
Other limited partnership interests	0	0
Other invested assets	0	87
Premiums, reinsurance and other receivables	0	0
Liabilities
PABs	0	0
Long-term debt	0	0
Other liabilities	0	0
Separate account liabilities	0	0
Estimated Fair Value [Member] \| Level 2
Assets
Mortgage loans	0	0
Policy loans	796	786
Real estate joint ventures	0	0
Other limited partnership interests	0	0
Other invested assets	2,270	1,752
Premiums, reinsurance and other receivables	94	15
Liabilities
PABs	0	0
Long-term debt	2,029	2,956
Other liabilities	609	310
Separate account liabilities	60,840	57,935
Estimated Fair Value [Member] \| Level 3
Assets
Mortgage loans	50,992	47,369
Policy loans	9,614	8,767
Real estate joint ventures	54	70
Other limited partnership interests	819	1,013
Other invested assets	220	176
Premiums, reinsurance and other receivables	14,607	14,906
Liabilities
PABs	75,481	72,236
Long-term debt	268	0
Other liabilities	20,133	19,787
Separate account liabilities	0	0
Carrying Value [Member]
Assets
Mortgage loans	48,751	45,686
Policy loans	8,491	8,421
Real estate joint ventures	30	47
Other limited partnership interests	635	865
Other invested assets	2,385	2,017
Premiums, reinsurance and other receivables	13,845	14,210
Liabilities
PABs	73,225	70,205
Long-term debt	1,897	2,655
Other liabilities	20,139	19,601
Separate account liabilities	$ 60,840	$ 57,935

Total amount of mortgage loans not valued on a recurring basis as of the balance sheet date.

No definition available.

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Namespace Prefix:	mlic_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of other investments not otherwise specified in the taxonomy.

+ References

No definition available.

+ Details

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Namespace Prefix:	mlic_
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- Definition

The fair value portion of investments in other limited partnership interests.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- Definition

Fair value portion of the liability of policyholders for deposits held under the terms of insurance contracts.

+ References

No definition available.

+ Details

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Namespace Prefix:	mlic_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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- Definition

Amount before allowance of loans made to policy and contract holders using their life insurance contracts' cash value or separate accounts as collateral. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums over and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(a)(7))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: 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(e))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph e

-Article 7

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Fair value portion of other liabilities.

+ References

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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- Definition

The book value of investments in real estate joint ventures including direct and indirect investments.

+ References

Reference 1: 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(d))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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Balance Type:	debit
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- Definition

Fair value portion of receivables, including, but not limited to, trade account receivables, note receivables, and loan receivables.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.3)

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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 3, 4

-Article 5

+ Details

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

Amount of liability for variable contracts that meet specified criteria that are backed by assets held in separate accounts and carried at the fair value of those assets.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 80

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6484091&loc=d3e19268-158472

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-Glossary Separate Account

-URI http://asc.fasb.org/extlink&oid=6524887

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 80

-Section 50

-Paragraph 1

-Subparagraph (b)(1)

-URI http://asc.fasb.org/extlink&oid=6484115&loc=d3e19393-158473

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 18

-Article 7

+ Details

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