Investments - Summary of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 4,328	$ 3,709
Gross unrealized gains	1	2
Gross unrealized losses	218	212
Estimated Fair Value	4,111	3,499
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,739	2,303
Gross unrealized gains	1	2
Gross unrealized losses	121	121
Estimated Fair Value	2,619	2,184
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	955	787
Gross unrealized gains	0	0
Gross unrealized losses	67	56
Estimated Fair Value	888	731
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	378	308
Gross unrealized gains	0	0
Gross unrealized losses	17	20
Estimated Fair Value	361	288
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	159	160
Gross unrealized gains	0	0
Gross unrealized losses	10	11
Estimated Fair Value	149	149
U.S. Treasury notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	51	106
Gross unrealized gains	0	0
Gross unrealized losses	0	1
Estimated Fair Value	51	105
Other
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	46	45
Gross unrealized gains	0	0
Gross unrealized losses	3	3
Estimated Fair Value	$ 43	$ 42

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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