Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2023	Dec. 31, 2022
Schedule of Premium Revenue by Health Plan Type [Line Items]
Risk adjustment payable	$ 163.0	$ 230.0
Risk adjustment receivable	246.0	135.0
Risk adjustment, net receivable	83.0
Risk adjustment, credit loss allowances	$ 41.0
Risk adjustment, net payable		$ 95.0
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, Risk Adjustment, Credit Loss Allowance

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

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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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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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