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Fair Value Measurements (Financial Instruments Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020
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Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | $ 91,071 |  | $ 90,699 |
Equity securities, at fair value | 4,405 |  | 4,027 |
Short-term investments | 3,735 |  | 4,345 |
Other investments, at fair value | 8,636 |  | 7,945 |
Securities lending collateral | 2,076 |  | 1,844 |
U.S. Treasury / Agency |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 2,527 |  | 2,670 |
Non-U.S. |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 26,166 |  | 26,354 |
Corporate and asset-backed securities |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 36,400 |  | 36,331 |
Mortgage-backed securities |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 19,437 |  | 18,470 |
Municipal |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 6,541 |  | 6,874 |
Fair Value, Recurring [Member] | Level 1 |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 2,044 |  | 2,148 |
Equity securities, at fair value | 4,330 |  | 3,954 |
Short-term investments | 2,279 |  | 2,866 |
Other investments, at fair value | 421 |  | 434 | [1]
Securities lending collateral | 0 |  | 0 |
Investment derivative instruments, assets | 69 |  | 35 |
Other Derivative Instruments Fair Value | 2 |  |  |
Separate Account Asset | 4,493 |  | 4,264 |
Total assets measured at fair value | 13,638 | [2] | 13,701 | [1]
Investment derivative instruments, liabilities | 79 |  | 52 |
Other derivative instruments, liability | 7 |  | 17 |
Liabilities Related to Investment Contracts, Fair Value Disclosure | 86 |  | 69 |
Fair Value, Recurring [Member] | Level 1 | U.S. Treasury / Agency |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 2,044 |  | 2,148 |
Fair Value, Recurring [Member] | Level 1 | Non-U.S. |  |  |  |
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 and asset-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 | 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 | Municipal |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 0 |  | 0 |
Fair Value, Recurring [Member] | Level 1 | GLB |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross | 0 |  | 0 | [3]
Fair Value, Recurring [Member] | Level 2 |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 86,714 |  | 86,372 |
Equity securities, at fair value | 0 |  | 0 |
Short-term investments | 1,455 |  | 1,474 |
Other investments, at fair value | 446 |  | 438 | [1]
Securities lending collateral | 2,076 |  | 1,844 |
Investment derivative instruments, assets | 0 |  | 0 |
Other Derivative Instruments Fair Value | 0 |  |  |
Separate Account Asset | 115 |  | 124 |
Total assets measured at fair value | 90,806 | [2] | 90,252 | [1]
Investment derivative instruments, liabilities | 0 |  | 0 |
Other derivative instruments, liability | 0 |  | 0 |
Liabilities Related to Investment Contracts, Fair Value Disclosure | 0 |  | 0 |
Fair Value, Recurring [Member] | Level 2 | U.S. Treasury / Agency |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 483 |  | 522 |
Fair Value, Recurring [Member] | Level 2 | Non-U.S. |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 25,516 |  | 25,808 |
Fair Value, Recurring [Member] | Level 2 | Corporate and asset-backed securities |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 34,788 |  | 34,758 |
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 | 19,386 |  | 18,410 |
Fair Value, Recurring [Member] | Level 2 | Municipal |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 6,541 |  | 6,874 |
Fair Value, Recurring [Member] | Level 2 | GLB |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross | 0 |  | 0 | [3]
Fair Value, Recurring [Member] | Level 3 |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 2,313 |  | 2,179 |
Equity securities, at fair value | 75 |  | 73 |
Short-term investments | 1 |  | 5 |
Other investments, at fair value | 10 |  | 10 | [1]
Securities lending collateral | 0 |  | 0 |
Investment derivative instruments, assets | 0 |  | 0 |
Other Derivative Instruments Fair Value | 0 |  |  |
Separate Account Asset | 0 |  | 0 |
Total assets measured at fair value | 2,399 | [2] | 2,267 | [1]
Investment derivative instruments, liabilities | 0 |  | 0 |
Other derivative instruments, liability | 0 |  | 0 |
Liabilities Related to Investment Contracts, Fair Value Disclosure | 760 |  | 1,089 |
Fair Value, Recurring [Member] | Level 3 | U.S. Treasury / 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 | Non-U.S. |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 650 |  | 546 |
Fair Value, Recurring [Member] | Level 3 | Corporate and asset-backed securities |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 1,612 |  | 1,573 |
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 | 51 |  | 60 |
Fair Value, Recurring [Member] | Level 3 | Municipal |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 0 |  | 0 |
Fair Value, Recurring [Member] | Level 3 | GLB |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross | 760 |  | 1,089 | [3]
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 | 91,071 |  | 90,699 |
Equity securities, at fair value | 4,405 |  | 4,027 |
Short-term investments | 3,735 |  | 4,345 |
Other investments, at fair value | 877 |  | 882 | [1]
Securities lending collateral | 2,076 |  | 1,844 |
Investment derivative instruments, assets | 69 |  | 35 |
Other Derivative Instruments Fair Value | 2 |  |  |
Separate Account Asset | 4,608 |  | 4,388 |
Total assets measured at fair value | 106,843 | [2] | 106,220 | [1]
Investment derivative instruments, liabilities | 79 |  | 52 |
Other derivative instruments, liability | 7 |  | 17 |
Liabilities Related to Investment Contracts, Fair Value Disclosure | 846 |  | 1,158 |
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | U.S. Treasury / Agency |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 2,527 |  | 2,670 |
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Non-U.S. |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 26,166 |  | 26,354 |
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Corporate and asset-backed securities |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 36,400 |  | 36,331 |
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 | 19,437 |  | 18,470 |
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Municipal |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Available for sale, Fair Value | 6,541 |  | 6,874 |
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | GLB |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross | 760 |  | 1,089 | [3]
Investment Funds Limited Partnerships Partially Owned Investment Companies Fair Value [Member] |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Other investments, at fair value | 7,457 |  | 6,770 |
Policy Loans [Member] |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Other investments, at fair value | 232 |  | 233 |
Other Investments [Member] |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] |  |  |  |
Other investments, at fair value | $ 70 |  | $ 60 |
[1] | Excluded from the table above are partially-owned investments, investment funds, and limited partnerships of $6,770 million, policy loans of $233 million and other investments of $60 million at December 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 $7,457 million, policy loans of $232 million and other investments of $70 million at March 31, 2021 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.

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