Investments - Available-for-Sale Investments (Details) $ in Millions	Mar. 31, 2024 USD ($) position	Dec. 31, 2023 USD ($) position
Debt Securities, Available-for-sale [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 750	$ 386
In a Continuous Loss Position for Less than 12 Months, Gross Unrealized Losses	$ 7	$ 3
In a Continuous Loss Position for Less than 12 Months, Total Number of Positions | position	482	258
In a Continuous Loss Position for 12 Months or More, Estimated Fair Value	$ 2,323	$ 2,431
In a Continuous Loss Position for 12 Months or More, Gross Unrealized Losses	$ 119	$ 126
In a Continuous Loss Position for 12 Months or More, Total Number of Positions | position	1,228	1,261
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 596	$ 263
In a Continuous Loss Position for Less than 12 Months, Gross Unrealized Losses	$ 5	$ 1
In a Continuous Loss Position for Less than 12 Months, Total Number of Positions | position	345	160
In a Continuous Loss Position for 12 Months or More, Estimated Fair Value	$ 1,502	$ 1,553
In a Continuous Loss Position for 12 Months or More, Gross Unrealized Losses	$ 58	$ 64
In a Continuous Loss Position for 12 Months or More, Total Number of Positions | position	738	754
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 154	$ 123
In a Continuous Loss Position for Less than 12 Months, Gross Unrealized Losses	$ 2	$ 2
In a Continuous Loss Position for Less than 12 Months, Total Number of Positions | position	137	98
In a Continuous Loss Position for 12 Months or More, Estimated Fair Value	$ 485	$ 549
In a Continuous Loss Position for 12 Months or More, Gross Unrealized Losses	$ 42	$ 42
In a Continuous Loss Position for 12 Months or More, Total Number of Positions | position	267	283
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 0	$ 0
In a Continuous Loss Position for Less than 12 Months, Gross Unrealized Losses	$ 0	$ 0
In a Continuous Loss Position for Less than 12 Months, Total Number of Positions | position	0	0
In a Continuous Loss Position for 12 Months or More, Estimated Fair Value	$ 208	$ 195
In a Continuous Loss Position for 12 Months or More, Gross Unrealized Losses	$ 10	$ 12
In a Continuous Loss Position for 12 Months or More, Total Number of Positions | position	97	91
Municipal securities
Debt Securities, Available-for-sale [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 0	$ 0
In a Continuous Loss Position for Less than 12 Months, Gross Unrealized Losses	$ 0	$ 0
In a Continuous Loss Position for Less than 12 Months, Total Number of Positions | position	0	0
In a Continuous Loss Position for 12 Months or More, Estimated Fair Value	$ 112	$ 117
In a Continuous Loss Position for 12 Months or More, Gross Unrealized Losses	$ 7	$ 6
In a Continuous Loss Position for 12 Months or More, Total Number of Positions | position	110	116
Other
Debt Securities, Available-for-sale [Line Items]
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value	$ 0	$ 0
In a Continuous Loss Position for Less than 12 Months, Gross Unrealized Losses	$ 0	$ 0
In a Continuous Loss Position for Less than 12 Months, Total Number of Positions | position	0	0
In a Continuous Loss Position for 12 Months or More, Estimated Fair Value	$ 16	$ 17
In a Continuous Loss Position for 12 Months or More, Gross Unrealized Losses	$ 2	$ 2
In a Continuous Loss Position for 12 Months or More, Total Number of Positions | position	16	17