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Fair Value - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018
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Corporate Bonds and Other [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | $ 21,153 | $ 19,810
States, Municipalities and Political Subdivisions [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 10,578 | 10,748
Total Asset-Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 8,794 | 8,984
Total Fixed Maturities Available-for-Sale [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 40,525 | 39,542
Fixed Maturities Trading [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 77 | 157
Fixed Maturity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 40,602 | 39,699
Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 1,326 | 1,293
Fair Value, Measurements, Recurring [Member] | Payable to Brokers [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of liabilities, measured on a recurring basis | (14) | (23)
Fair Value, Measurements, Recurring [Member] | Receivables [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 3 | 11
Short Term and Other [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 4,403 | 3,758
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Payable to Brokers [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of liabilities, measured on a recurring basis | (14) | (23)
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds and Other [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 203 | 196
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Total Fixed Maturities Available-for-Sale [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 203 | 196
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Fixed Maturity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 203 | 196
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 697 | 704
Level 1 [Member] | Short Term and Other [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 3,199 | 2,647
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds and Other [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 20,697 | 19,392
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | States, Municipalities and Political Subdivisions [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 10,578 | 10,748
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Total Asset-Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 8,610 | 8,787
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Total Fixed Maturities Available-for-Sale [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 39,885 | 38,927
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Fixed Maturities Trading [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 72 | 151
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Fixed Maturity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 39,957 | 39,078
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 608 | 570
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Receivables [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 3 | 11
Level 2 [Member] | Short Term and Other [Member] | Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 1,204 | 1,111
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds and Other [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 253 | 222
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Total Asset-Backed [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 184 | 197
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Total Fixed Maturities Available-for-Sale [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 437 | 419
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Fixed Maturities Trading [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 5 | 6
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Fixed Maturity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | 442 | 425
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair value of assets, measured on a recurring basis | $ 21 | $ 19

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