Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2024	Dec. 31, 2023
Schedule of Premium Revenue by Health Plan Type [Line Items]
Amounts due to government agencies, risk adjustment and Part D risk sharing		$ 82	$ 66
Risk adjustment payable		207	201
Risk adjustment receivable		282	241
Risk adjustment, net receivable		$ 75	$ 40
Risk adjustment, credit loss allowance	$ 41
Maturity period (less than)		15 years
Structured Securities
Schedule of Premium Revenue by Health Plan Type [Line Items]
Average maturity period (less than)		15 years

Affordable Care Act, Premium Stabilization Program, Credit Loss Allowance

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Affordable Care Act, Premium Stabilization Program, Risk Adjustment, Net Receivable

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Affordable Care Act, Premium Stabilization Program, Risk Adjustment, (Payable)

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Affordable Care Act, Premium Stabilization Program, Risk Adjustment, Receivable

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Amounts Due to Government Agencies, Risk Adjustment and Part D risk sharing

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Investments, Average Maturity Period

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Investments, Maturity Period

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Schedule of premium revenue by health plan type.

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