Investments (Components of Net Investment Gains Losses) (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Marketable Securities, Gain (Loss) [Abstract]
Fixed maturity securities — net gains (losses) on sales and disposals	$ (99,000,000)us-gaap_GainLossOnSaleOfDebtInvestments	$ 177,000,000us-gaap_GainLossOnSaleOfDebtInvestments	$ 16,000,000us-gaap_GainLossOnSaleOfDebtInvestments
Equity securities — net gains (losses) on sales and disposals	42,000,000	6,000,000	15,000,000
Other net investment gains (losses):
Trading and FVO securities — FVO general account securities	1,000,000	11,000,000	11,000,000
Mortgage loans	(36,000,000)us-gaap_GainLossOnSalesOfLoansNet	31,000,000us-gaap_GainLossOnSalesOfLoansNet	84,000,000us-gaap_GainLossOnSalesOfLoansNet
Real estate and real estate joint ventures	252,000,000	(15,000,000)	(27,000,000)
Other limited partnership interests	(69,000,000)	(41,000,000)	(35,000,000)
Other investment portfolio gains (losses)	(108,000,000)us-gaap_GainLossOnSaleOfInvestments	5,000,000us-gaap_GainLossOnSaleOfInvestments	(192,000,000)us-gaap_GainLossOnSaleOfInvestments
Subtotal — investment portfolio gains (losses)	(64,000,000)mlic_InvestmentPortfolioGainsLosses	27,000,000mlic_InvestmentPortfolioGainsLosses	(327,000,000)mlic_InvestmentPortfolioGainsLosses
FVO CSEs - changes in estimated fair value subsequent to consolidation:
Securities	0	2,000,000	0
Long-term debt — related to securities	(1,000,000)	(2,000,000)	(7,000,000)
Non-investment portfolio gains (losses)	208,000,000us-gaap_GainLossOnSaleOfOtherAssets	21,000,000us-gaap_GainLossOnSaleOfOtherAssets	4,000,000us-gaap_GainLossOnSaleOfOtherAssets
Subtotal FVO CSEs and non-investment portfolio gains (losses)	207,000,000	21,000,000	(3,000,000)
Total net investment gains (losses)	143,000,000us-gaap_GainLossOnInvestments	48,000,000us-gaap_GainLossOnInvestments	(330,000,000)us-gaap_GainLossOnInvestments
Fixed maturity securities
Marketable Securities, Gain (Loss) [Abstract]
OTTI losses (1)	(26,000,000)	(128,000,000)	(192,000,000)
Net investment gains (losses)	(125,000,000)	49,000,000	(176,000,000)
Consumer
Marketable Securities, Gain (Loss) [Abstract]
OTTI losses (1)	(6,000,000)	(12,000,000)	(19,000,000)
Utility
Marketable Securities, Gain (Loss) [Abstract]
OTTI losses (1)	0	(48,000,000)	(29,000,000)
Finance
Marketable Securities, Gain (Loss) [Abstract]
OTTI losses (1)	0	(4,000,000)	(21,000,000)
Communications
Marketable Securities, Gain (Loss) [Abstract]
OTTI losses (1)	0	(2,000,000)	(18,000,000)
Industrial
Marketable Securities, Gain (Loss) [Abstract]
OTTI losses (1)	0	0	(4,000,000)
Transportation
Marketable Securities, Gain (Loss) [Abstract]
OTTI losses (1)	0	0	(1,000,000)
Corporate fixed maturity securities [Member]
Marketable Securities, Gain (Loss) [Abstract]
OTTI losses (1)	(6,000,000)	(66,000,000)	(92,000,000)
RMBS
Marketable Securities, Gain (Loss) [Abstract]
OTTI losses (1)	(20,000,000)	(62,000,000)	(70,000,000)
CMBS
Marketable Securities, Gain (Loss) [Abstract]
OTTI losses (1)	0	0	(28,000,000)
ABS
Marketable Securities, Gain (Loss) [Abstract]
OTTI losses (1)	0	0	(2,000,000)
Equity securities
Marketable Securities, Gain (Loss) [Abstract]
OTTI losses (1)	(21,000,000)	(19,000,000)	(7,000,000)
Net investment gains (losses)	21,000,000	(13,000,000)	8,000,000
Non-redeemable preferred stock
Marketable Securities, Gain (Loss) [Abstract]
OTTI losses (1)	(16,000,000)	(17,000,000)	0
Common stock
Marketable Securities, Gain (Loss) [Abstract]
OTTI losses (1)	$ (5,000,000)	$ (2,000,000)	$ (7,000,000)

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The net gain or loss incurred on assets and liabilities held by consolidated securitizations entities and all other gains and losses.

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The net gain or loss resulting from the change in fair value of commercial mortgage loans held by consolidated securitization entities not required to be reported separately in the statement of income for which the Company has elected the fair value option.

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The net change during the reporting period in the aggregate market value of equity or debt securities held by consolidated securitization entities that are purchased and held principally for the purpose of selling them in the near future.

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This item represents the subtotal of gains (losses)on the investment portfolio.

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Trading and other securities, fair value option, general account securities changes in estimated fair value.

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Amount of gain (loss) on sale or disposal of an equity method investment.

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This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments.

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The difference between the carrying value and the sale price of debt securities, not separately or otherwise categorized as trading, available-for-sale, or held-to-maturity. This element refers to the gain (loss) included in earnings and not to the cash proceeds of the sale.

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The difference between the carrying value and the sale price of equity securities, not separately or otherwise categorized as trading or available-for-sale. This element includes investments in which the entity holds a small ownership stake (generally, less than 20% of the shares outstanding) and cannot exert significant influence.

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The net realized gain (loss) on investments sold during the period, not including gains (losses) on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, which, for cash flow reporting, is a component of proceeds from investing activities.

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Amount of gain (loss) on sale or disposal of other assets.

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The net gain (loss) resulting from a sale of loans, including adjustments to record loans classified as held-for-sale at the lower-of-cost-or-market and fair value adjustments to loan held for investment purposes.

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The net gain (loss) resulting from sales and other disposals of real estate owned for investment purposes.

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The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

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The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

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