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Fair Value Measurements (Financial Instruments Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | [2],[3] | Dec. 31, 2014 | [3]
--- | --- | --- | --- | --- | --- | ---
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Available for sale, Fair Value | $ 78,939 |  | $ 80,115 |  |  |
Equity securities, at fair value (cost – $737 and $706) | 937 |  | 814 |  |  |
Short-term investments | 3,561 |  | 3,002 |  |  |
Other investments (cost – $4,417 and $4,270) | 4,672 |  | 4,519 |  |  |
Securities lending collateral | 1,737 |  | 1,092 |  |  |
Investment Funds Limited Partnerships Partially Owned Investment Companies Fair Value [Member] |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Other investments (cost – $4,417 and $4,270) | 3,623 |  | 3,626 |  |  |
Other Investments [Member] |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Other investments (cost – $4,417 and $4,270) | 15 |  | 25 |  |  |
Fair Value, Measurements, Recurring [Member] | Level 1 |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Available for sale, Fair Value | 3,129 |  | 2,175 |  |  |
Equity securities, at fair value (cost – $737 and $706) | 893 |  | 773 |  |  |
Short-term investments | 2,309 |  | 1,757 |  |  |
Other investments (cost – $4,417 and $4,270) | 466 |  | 384 |  |  |
Investment derivative instruments, assets | 18 |  | 31 |  |  |
Other derivative instruments, assets | 1 |  | 3 |  |  |
Separate Account Assets | 2,635 |  | 1,784 |  |  |
Total assets measured at fair value | 9,451 |  | 6,907 |  |  |
Investment derivative instruments, liability | 30 |  | 54 |  |  |
Other derivative instruments, liability | 21 |  |  |  |  |
Total liabilities measured at fair value | 51 |  | 54 |  |  |
Fair Value, Measurements, Recurring [Member] | Level 2 |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Available for sale, Fair Value | 74,602 |  | 77,140 |  |  |
Short-term investments | 1,252 |  | 1,220 |  |  |
Other investments (cost – $4,417 and $4,270) | 305 |  | 259 |  |  |
Securities lending collateral | 1,737 |  | 1,092 |  |  |
Separate Account Assets | 99 |  | 95 |  |  |
Total assets measured at fair value | 77,995 |  | 79,806 |  |  |
Fair Value, Measurements, Recurring [Member] | Level 3 |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Available for sale, Fair Value | 1,208 |  | 800 |  |  |
Equity securities, at fair value (cost – $737 and $706) | 44 |  | 41 |  |  |
Short-term investments |  |  | 25 |  |  |
Other investments (cost – $4,417 and $4,270) | 263 |  | 225 |  |  |
Total assets measured at fair value | 1,515 |  | 1,091 |  |  |
Other derivative instruments, liability | 2 |  | 13 |  |  |
Total liabilities measured at fair value | 206 |  | 572 |  |  |
Fair Value, Measurements, Recurring [Member] | US Treasury and Government [Member] | Level 1 |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Available for sale, Fair Value | 3,129 |  | 2,175 |  |  |
Fair Value, Measurements, Recurring [Member] | US Treasury and Government [Member] | Level 2 |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Available for sale, Fair Value | 569 |  | 695 |  |  |
Fair Value, Measurements, Recurring [Member] | Foreign | Level 2 |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Available for sale, Fair Value | 20,937 |  | 21,366 |  |  |
Fair Value, Measurements, Recurring [Member] | Foreign | Level 3 |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Available for sale, Fair Value | 93 |  | 74 |  |  |
Fair Value, Measurements, Recurring [Member] | Corporate securities | Level 2 |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Available for sale, Fair Value | 22,959 |  | 23,468 |  |  |
Fair Value, Measurements, Recurring [Member] | Corporate securities | Level 3 |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Available for sale, Fair Value | 1,037 |  | 681 |  |  |
Fair Value, Measurements, Recurring [Member] | Mortgage backed-securities | Level 2 |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Available for sale, Fair Value | 15,212 |  | 13,962 |  |  |
Fair Value, Measurements, Recurring [Member] | Mortgage backed-securities | Level 3 |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Available for sale, Fair Value | 78 |  | 45 |  |  |
Fair Value, Measurements, Recurring [Member] | States, municipalities, and political subdivisions | Level 2 |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Available for sale, Fair Value | 14,925 |  | 17,649 |  |  |
Guaranteed Minimum Income Benefit |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value | 204 | [1] | 559 | [1],[2] | $ 609 | $ 406
Guaranteed Minimum Income Benefit | Level 3 |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value | 204 | [4] | 559 |  |  |
Guaranteed Minimum Income Benefit | Fair Value, Measurements, Recurring [Member] | Level 3 |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |
Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross | $ 204 |  | $ 559 |  |  |
[1] | Our GLB reinsurance product meets the definition of a derivative instrument for accounting purposes and is accordingly carried at fair value. Excluded from the table above is the portion of the GLB derivative liability classified as Future policy benefits in the Consolidated balance sheets. Refer to Note 5 c) for additional information.
[2] | Our GLB reinsurance product meets the definition of a derivative instrument for accounting purposes and is accordingly carried at fair value. Excluded from the table above is the portion of the GLB derivative liability classified as Future policy benefits in the Consolidated balance sheets. The liability for GLB reinsurance was $853 million at December 31, 2016 and $888 million at December 31, 2015, which includes a fair value derivative adjustment of $559 million and $609 million, respectively.
[3] | Our GLB reinsurance product meets the definition of a derivative instrument for accounting purposes and is accordingly carried at fair value. Excluded from the table above is the portion of the GLB derivative liability classified as Future policy benefits in the Consolidated balance sheets. The liability for GLB reinsurance was $888 million at December 31, 2015 and $663 million at December 31, 2014, which includes a fair value derivative adjustment of $609 million and $406 million, respectively.
[4] | Discussion of the most significant inputs used in the fair value measurement of GLB and the sensitivity of those assumptions is included within Note 4 a) Guaranteed living benefits.

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