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Fair Value - Composition of Investment Portfolio by Major Security Type (Detail) - USD ($)
$ in Millions
Sep. 30, 2025
Dec. 31, 2024
Sep. 30, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities $ 90,024 $ 75,947 $ 75,168
Equity securities 4,485 4,303 4,232
Investments, fair value 94,509 80,250 79,400
Fair value 6,398 6,173 6,498
Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 438 728 735
Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 4,047 3,575 3,497
Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 4,013 3,550 3,474
Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 34 25 23
Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 88,509 75,332 74,411
Total fixed maturities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 51,339 45,988 44,429
Total fixed maturities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 3,275 2,778 2,601
Total fixed maturities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 16 16 16
Total fixed maturities | Corporate and other debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 18,817 13,954 15,036
Total fixed maturities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 3,000 1,601 1,424
Total fixed maturities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 5,686 4,352 4,291
Total fixed maturities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 6,376 6,643 6,614
Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 51,339 45,988 44,429
Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 3,275 2,778 2,601
Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 16 16 16
Total fixed maturities | Debt Securities | Corporate and other debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 18,817 13,954 15,036
Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 3,000 1,601 1,424
Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 5,686 4,352 4,291
Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 6,376 6,643 6,614
Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 1,515 615 757
Fair Value, Inputs, Level 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 52,753 46,601 45,184
Equity securities 4,004 3,527 3,452
Investments, fair value 56,757 50,128 48,636
Fair value 0 0 0
Fair Value, Inputs, Level 1 | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0 0 0
Fair Value, Inputs, Level 1 | Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 4,004 3,527 3,452
Fair Value, Inputs, Level 1 | Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 4,004 3,527 3,452
Fair Value, Inputs, Level 1 | Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0 0 0
Fair Value, Inputs, Level 1 | Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 51,339 45,988 44,429
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 51,339 45,988 44,429
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0 0 0
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0 0 0
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | Corporate and other debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0 0 0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0 0 0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0 0 0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0 0 0
Fair Value, Inputs, Level 1 | Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 1,414 613 755
Fair Value, Inputs, Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 37,266 29,341 29,981
Equity securities 378 676 683
Investments, fair value 37,644 30,017 30,664
Fair value 6,398 6,173 6,498
Fair Value, Inputs, Level 2 | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 378 676 683
Fair Value, Inputs, Level 2 | Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0 0 0
Fair Value, Inputs, Level 2 | Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0 0 0
Fair Value, Inputs, Level 2 | Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0 0 0
Fair Value, Inputs, Level 2 | Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 37,165 29,339 29,979
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0 0 0
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 3,275 2,778 2,601
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 16 16 16
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | Corporate and other debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 18,812 13,949 15,033
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 3,000 1,601 1,424
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 5,686 4,352 4,291
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 6,376 6,643 6,614
Fair Value, Inputs, Level 2 | Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 101 2 2
Fair Value, Inputs, Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 5 5 3
Equity securities 103 100 97
Investments, fair value 108 105 100
Fair value 0 0 0
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 60 52 52
Fair Value, Inputs, Level 3 | Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 43 48 45
Fair Value, Inputs, Level 3 | Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 9 23 22
Fair Value, Inputs, Level 3 | Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 34 25 23
Fair Value, Inputs, Level 3 | Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 5 5 3
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0 0 0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0 0 0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0 0 0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | Corporate and other debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 5 5 3
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0 0 0
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0 0 0
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0 0 0
Fair Value, Inputs, Level 3 | Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0 0 0
Carrying (Reported) Amount, Fair Value Disclosure      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 89,762 77,741 75,352
Equity securities 1,262 1,501 1,493
Investments, fair value 91,024 79,242 76,845
Long-term debt 6,896 6,893 6,892
Carrying (Reported) Amount, Fair Value Disclosure | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 454 756 760
Carrying (Reported) Amount, Fair Value Disclosure | Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 808 745 733
Carrying (Reported) Amount, Fair Value Disclosure | Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 774 720 710
Carrying (Reported) Amount, Fair Value Disclosure | Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 34 25 23
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 88,247 77,126 74,595
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 51,021 47,103 44,231
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 3,320 2,893 2,681
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 16 16 17
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | Corporate and other debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 18,621 14,111 14,970
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 2,979 1,600 1,404
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 5,916 4,721 4,664
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 6,374 6,682 6,628
Carrying (Reported) Amount, Fair Value Disclosure | Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities $ 1,515 $ 615 $ 757