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Fair Value Measurements (Financial Instruments Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions | Mar. 31, 2020 | Dec. 31, 2019 | Mar. 31, 2019 | Dec. 31, 2018
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Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | $ 81,530 |  | $ 85,488 |  |  |
Equity securities, at fair value |  | 2,068 |  | 812 |  |  |
Short-term investments |  | 3,586 |  | 4,291 |  |  |
Other Investments |  | 6,075 |  | 6,062 |  |  |
Securities lending collateral |  | 1,342 |  | 994 |  |  |
U.S. Treasury and agency |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 3,013 |  | 3,283 |  |  |
Foreign |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 22,844 |  | 23,707 |  |  |
Corporate securities |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 30,668 |  | 31,791 |  |  |
Mortgage-backed securities |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 17,885 |  | 19,192 |  |  |
States, municipalities, and political subdivisions |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 7,120 |  | 7,515 |  |  |
Investment Funds Limited Partnerships Partially Owned Investment Companies Fair Value [Member] |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Other Investments |  | 5,091 |  | 4,921 |  |  |
Other Investments [Member] |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Other Investments |  | 126 |  | 95 |  |  |
Policy Loans [Member] |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Other Investments |  | 228 |  |  |  |  |
Fair Value, Recurring [Member] | Level 1 |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 2,400 |  | 2,664 |  |  |
Equity securities, at fair value |  | 2,001 |  | 728 |  |  |
Short-term investments |  | 2,165 |  | 2,803 |  |  |
Other Investments |  | 285 | [1] | 412 | [2] |  |
Securities lending collateral |  | 0 |  | 0 |  |  |
Investment derivative instruments, assets |  | 64 |  | 24 |  |  |
Other Derivative Instruments Fair Value |  | 2 |  | 2 |  |  |
Separate Account Asset |  | 2,778 |  | 3,437 |  |  |
Total assets measured at fair value |  | 9,695 | [1] | 10,070 | [2] |  |
Investment derivative instruments, liabilities |  | 141 |  | 93 |  |  |
Other derivative instruments, liability |  |  |  | 13 |  |  |
Liabilities Related to Investment Contracts, Fair Value Disclosure |  | 141 |  | 106 |  |  |
Fair Value, Recurring [Member] | Level 1 | U.S. Treasury and agency |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 2,400 |  | 2,664 |  |  |
Fair Value, Recurring [Member] | Level 1 | Foreign |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 0 |  | 0 |  |  |
Fair Value, Recurring [Member] | Level 1 | Corporate securities |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 0 |  | 0 |  |  |
Fair Value, Recurring [Member] | Level 1 | Mortgage-backed securities |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 0 |  | 0 |  |  |
Fair Value, Recurring [Member] | Level 1 | States, municipalities, and political subdivisions |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 0 |  | 0 |  |  |
Fair Value, Recurring [Member] | Level 2 |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 77,103 |  | 80,864 |  |  |
Equity securities, at fair value |  | 0 |  | 15 |  |  |
Short-term investments |  | 1,420 |  | 1,482 |  |  |
Other Investments |  | 335 | [1] | 377 | [2] |  |
Securities lending collateral |  | 1,342 |  | 994 |  |  |
Investment derivative instruments, assets |  | 0 |  | 0 |  |  |
Other Derivative Instruments Fair Value |  | 0 |  | 0 |  |  |
Separate Account Asset |  | 130 |  | 136 |  |  |
Total assets measured at fair value |  | 80,330 | [1] | 83,868 | [2] |  |
Investment derivative instruments, liabilities |  | 0 |  | 0 |  |  |
Other derivative instruments, liability |  |  |  | 0 |  |  |
Liabilities Related to Investment Contracts, Fair Value Disclosure |  | 0 |  | 0 |  |  |
Fair Value, Recurring [Member] | Level 2 | U.S. Treasury and agency |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 613 |  | 619 |  |  |
Fair Value, Recurring [Member] | Level 2 | Foreign |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 22,379 |  | 23,258 |  |  |
Fair Value, Recurring [Member] | Level 2 | Corporate securities |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 29,166 |  | 30,340 |  |  |
Fair Value, Recurring [Member] | Level 2 | Mortgage-backed securities |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 17,825 |  | 19,132 |  |  |
Fair Value, Recurring [Member] | Level 2 | States, municipalities, and political subdivisions |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 7,120 |  | 7,515 |  |  |
Fair Value, Recurring [Member] | Level 3 |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 2,027 |  | 1,960 |  |  |
Equity securities, at fair value |  | 67 |  | 69 |  |  |
Short-term investments |  | 1 |  | 6 |  |  |
Other Investments |  | 10 | [1] | 10 | [2] |  |
Securities lending collateral |  | 0 |  | 0 |  |  |
Investment derivative instruments, assets |  | 0 |  | 0 |  |  |
Other Derivative Instruments Fair Value |  | 0 |  | 0 |  |  |
Separate Account Asset |  | 0 |  | 0 |  |  |
Total assets measured at fair value |  | 2,105 | [1] | 2,045 | [2] |  |
Investment derivative instruments, liabilities |  | 0 |  | 0 |  |  |
Other derivative instruments, liability |  |  |  | 0 |  |  |
Liabilities Related to Investment Contracts, Fair Value Disclosure |  | 1,141 |  | 456 |  |  |
Fair Value, Recurring [Member] | Level 3 | U.S. Treasury and agency |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 0 |  | 0 |  |  |
Fair Value, Recurring [Member] | Level 3 | Foreign |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 465 |  | 449 |  |  |
Fair Value, Recurring [Member] | Level 3 | Corporate securities |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 1,502 |  | 1,451 |  |  |
Fair Value, Recurring [Member] | Level 3 | Mortgage-backed securities |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 60 |  | 60 |  |  |
Fair Value, Recurring [Member] | Level 3 | States, municipalities, and political subdivisions |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 0 |  | 0 |  |  |
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 81,530 |  | 85,488 |  |  |
Equity securities, at fair value |  | 2,068 |  | 812 |  |  |
Short-term investments |  | 3,586 |  | 4,291 |  |  |
Other Investments |  | 630 | [1] | 799 | [2] |  |
Securities lending collateral |  | 1,342 |  | 994 |  |  |
Investment derivative instruments, assets |  | 64 |  | 24 |  |  |
Other Derivative Instruments Fair Value |  | 2 |  | 2 |  |  |
Separate Account Asset |  | 2,908 |  | 3,573 |  |  |
Total assets measured at fair value |  | 92,130 | [1] | 95,983 | [2] |  |
Investment derivative instruments, liabilities |  | 141 |  | 93 |  |  |
Other derivative instruments, liability |  |  |  | 13 |  |  |
Liabilities Related to Investment Contracts, Fair Value Disclosure |  | 1,282 |  | 562 |  |  |
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | U.S. Treasury and agency |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 3,013 |  | 3,283 |  |  |
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Foreign |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 22,844 |  | 23,707 |  |  |
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Corporate securities |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 30,668 |  | 31,791 |  |  |
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Mortgage-backed securities |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 17,885 |  | 19,192 |  |  |
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | States, municipalities, and political subdivisions |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Available for sale, Fair Value |  | 7,120 |  | 7,515 |  |  |
Guaranteed Minimum Income Benefit [Member] |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value |  | 1,141 |  | 456 |  | $ 338 | $ 452
Guaranteed Minimum Income Benefit [Member] | Fair Value, Recurring [Member] | Level 1 |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value |  | 0 | [3] | 0 | [4] |  |
Guaranteed Minimum Income Benefit [Member] | Fair Value, Recurring [Member] | Level 2 |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value |  | 0 | [3] | 0 | [4] |  |
Guaranteed Minimum Income Benefit [Member] | Fair Value, Recurring [Member] | Level 3 |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value | [5] | 1,141 |  | 456 | [4] |  |
Guaranteed Minimum Income Benefit [Member] | Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] |  |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value |  | $ 1,141 | [3] | $ 456 | [4] |  |
[1] | Excluded from the table above are partially-owned investments, investment funds, and limited partnerships of $5,091 million, policy loans of $228 million and other investments of $126 million at March 31, 2020 measured using NAV as a practical expedient.
[2] | Excluded from the table above are partially-owned investments, investment funds, and limited partnerships of $4,921 million and other investments of $95 million at December 31, 2019 measured using NAV as a practical expedient.
[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.
[4] | 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.
[5] | The weighted average lapse and annuitization rates are determined by weighting each treaty's rates by the GLB contracts fair value.

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