Investments (Components of Net Investment Gains Losses) (Details) (USD $) In Millions, unless otherwise specified	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	$ 598us-gaap_GainLossOnSaleOfDebtInvestments	$ 561us-gaap_GainLossOnSaleOfDebtInvestments	$ 253us-gaap_GainLossOnSaleOfDebtInvestments
Equity securities — net gains (losses) on sales and disposals	101	31	38
Other net investment gains (losses):
FVO and trading securities — FVO general account securities	9	15	17
Mortgage Loans	(36)us-gaap_GainLossOnSalesOfLoansNet	22us-gaap_GainLossOnSalesOfLoansNet	57us-gaap_GainLossOnSalesOfLoansNet
Real estate and real estate joint ventures	222	(19)	(36)
Other limited partnership interests	(78)	(48)	(36)
Other investment portfolio gains (losses)	(110)us-gaap_GainLossOnSaleOfInvestments	22us-gaap_GainLossOnSaleOfInvestments	(151)us-gaap_GainLossOnSaleOfInvestments
Subtotal - investment portfolio gains (losses)	610met_InvestmentPortfolioGainsLosses	392met_InvestmentPortfolioGainsLosses	(209)met_InvestmentPortfolioGainsLosses
FVO CSEs - changes in estimated fair value subsequent to consolidation:
Commercial mortgage loans	(13)	(52)	7
Securities	0	2	0
Long-term debt — related to commercial mortgage loans	19	85	25
Long-term debt — related to securities	(1)	(2)	(7)
Non-investment portfolio gains (losses)	(812)us-gaap_GainLossOnSaleOfOtherAssets	(264)us-gaap_GainLossOnSaleOfOtherAssets	(168)us-gaap_GainLossOnSaleOfOtherAssets
Subtotal FVO CSEs and non-investment portfolio gains (losses)	(807)	(231)	(143)
Total net investment gains (losses)	(197)us-gaap_GainLossOnInvestments	161us-gaap_GainLossOnInvestments	(352)us-gaap_GainLossOnInvestments
Fixed Maturity Securities
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	(60)	(166)	(317)
Net investment gains (losses)	538	395	(64)
Consumer
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	(7)	(11)	(19)
Transportation
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	(2)	(3)	(17)
Utility
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	0	(48)	(61)
Finance
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	0	(10)	(32)
Communications
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	0	(2)	(19)
Technology
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	0	0	(6)
Industrial
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	0	0	(5)
Corporate fixed maturity securities [Member]
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	(9)	(74)	(159)
RMBS
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	(31)	(80)	(97)
CMBS (1)
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	(13)	(12)	(51)
ABS
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	(7)	0	(9)
State and political subdivision
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	0	0	(1)
Equity securities
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	(36)	(26)	(34)
Net investment gains (losses)	65	5	4
Non-redeemable preferred stock
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	(23)	(20)	0
Common stock
Marketable Securities, Gain (Loss) [Abstract]
Total OTTI losses recognized in earnings	$ (13)	$ (6)	$ (34)

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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 gain or loss resulting from the sale or disposal of long-term debt relating to mortgage loans of consolidated securitization entities not required to be reported separately in the statement of income for which the Company has elected the fair 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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