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Investments - Summary of Aggregate Contractual or Notional Amounts and Estimated Fair Values Related to Derivative Financial Instruments (Detail) - USD ($) $ in Millions | Jun. 30, 2019 | Dec. 31, 2018
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Embedded Derivative on Funds Withheld Liability [Member] |  |
Derivative [Line Items] |  |
Contractual/Notional Amount |  | $ 172
Estimated Fair Value, Asset |  | 4
With Hedge Designation [Member] | Interest Rate Swaps [Member] |  |
Derivative [Line Items] |  |
Contractual/Notional Amount | $ 540 | 500
Estimated Fair Value, Asset |  | 11
Estimated Fair Value, (Liability) | (9) |
Without Hedge Designation [Member] | Equity Markets, Options - Purchased [Member] |  |
Derivative [Line Items] |  |
Contractual/Notional Amount | 303 | 213
Estimated Fair Value, Asset | 5 | 18
Without Hedge Designation [Member] | Equity Markets, Options - Written [Member] |  |
Derivative [Line Items] |  |
Contractual/Notional Amount | 95 | 239
Estimated Fair Value, (Liability) | (3) | (17)
Without Hedge Designation [Member] | Commodity Futures [Member] |  |
Derivative [Line Items] |  |
Contractual/Notional Amount | 11 |
Without Hedge Designation [Member] | Commodity Futures [Member] | Long [Member] |  |
Derivative [Line Items] |  |
Contractual/Notional Amount |  | $ 32
Without Hedge Designation [Member] | Embedded Derivative on Funds Withheld Liability [Member] |  |
Derivative [Line Items] |  |
Contractual/Notional Amount | 172 |
Estimated Fair Value, (Liability) | $ (8) |

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