Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Details) - USD ($) $ in Millions	3 Months Ended
Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	$ 949	$ 1,032
Debt Securities, Available-for-sale, Allowance for Credit Loss	1	3	$ 2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	54	40
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	235	187
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	6	4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	714	844
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	43	68
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	49	71
Fixed maturity securities, at fair value	2,073
Debt Securities, Available-for-sale, Amortized Cost	2,069	2,197
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Period Increase (Decrease)	0
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	0	4
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	0	4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Fixed maturity securities, at fair value	305	426
Debt Securities, Available-for-sale, Amortized Cost	305	426
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	949	1,028
Debt Securities, Available-for-sale, Allowance for Credit Loss	1	3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	54	40
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	234	184
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	6	4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	714	844
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	43	68
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	49	71
Fixed maturity securities, at fair value	1,740	1,710
Debt Securities, Available-for-sale, Amortized Cost	1,737	1,744
Fixed Income Securities [Member] \| Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	18	23
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	2	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	3	7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	15	16
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1	1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1	1
Fixed maturity securities, at fair value	84	72
Debt Securities, Available-for-sale, Amortized Cost	83	72
Fixed Income Securities [Member] \| Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	566	584
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	5	4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	118	75
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	3	2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	448	509
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	30	43
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	34	44
Fixed maturity securities, at fair value	779	745
Debt Securities, Available-for-sale, Amortized Cost	808	785
Fixed Income Securities [Member] \| Foreign Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	68	64
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	16	8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	52	56
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	6	6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	6	6
Fixed maturity securities, at fair value	115	100
Debt Securities, Available-for-sale, Amortized Cost	120	105
Fixed Income Securities [Member] \| US Government Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	43	63
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	18	27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	24	37
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1	2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2	4
Fixed maturity securities, at fair value	91	82
Debt Securities, Available-for-sale, Amortized Cost	92	85
Fixed Income Securities [Member] \| Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	109	104
Debt Securities, Available-for-sale, Allowance for Credit Loss	1	3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	32	28
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	7	6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	102	98
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4	14
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	5	14
Fixed maturity securities, at fair value	281	250
Debt Securities, Available-for-sale, Amortized Cost	255	239
Fixed Income Securities [Member] \| Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	28	95
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	28	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	93
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	1
Fixed maturity securities, at fair value	113	139
Debt Securities, Available-for-sale, Amortized Cost	113	139
Fixed Income Securities [Member] \| Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	114	92
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	13	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	40	57
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	73	35
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1	1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2	1
Fixed maturity securities, at fair value	252	303
Debt Securities, Available-for-sale, Amortized Cost	242	301
Fixed Income Securities [Member] \| Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss Position	4	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	4	3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Fixed maturity securities, at fair value	24
Debt Securities, Available-for-sale, Amortized Cost	24
Fixed Income Securities [Member] \| Collateralized Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Fixed maturity securities, at fair value	24	19
Debt Securities, Available-for-sale, Amortized Cost	24	19
Fixed Income Investments And Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Allowance for Credit Loss	1	3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	54	40
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	49	71
Fixed maturity securities, at fair value	2,046	2,135
Debt Securities, Available-for-sale, Amortized Cost	2,042	2,170
Collateral Pledged [Member] \| Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Fixed maturity securities, at fair value	27	27
Debt Securities, Available-for-sale, Amortized Cost	27	0
Collateral Pledged [Member] \| Fixed Income Securities [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Fixed maturity securities, at fair value	0	27
Debt Securities, Available-for-sale, Amortized Cost	0	27
Collateral Pledged [Member] \| Fixed Income Securities [Member] \| US Treasury and Government
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Fixed maturity securities, at fair value	27	0
Debt Securities, Available-for-sale, Amortized Cost	27	0
Asset Pledged as Collateral [Member] \| Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Securities, Available-for-sale, Amortized Cost	$ 27	$ 27

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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