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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

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Fair Value Measurement - Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025	Sep. 30, 2025	Dec. 31, 2024
Financial assets:
Cash, cash equivalents, and restricted cash	$ 326.8	$ 250.1	$ 272.2	$ 232.8
Fixed maturities available-for-sale	436.3	293.4
Short-term investments	62.8	152.5
Total financial assets	825.9	696.0
U.S. government and agencies
Financial assets:
Fixed maturities available-for-sale	96.2	75.7
Short-term investments	51.5	103.5
States and other territories
Financial assets:
Fixed maturities available-for-sale	6.8	8.2
Corporate securities
Financial assets:
Fixed maturities available-for-sale	242.2	146.4
Short-term investments	11.3	49.0
Residential mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	57.0	37.7
Commercial mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	2.9	5.0
Asset backed securities
Financial assets:
Fixed maturities available-for-sale	31.2	20.4
Level 1
Financial assets:
Cash, cash equivalents, and restricted cash	326.8	250.1
Fixed maturities available-for-sale	96.2	75.7
Short-term investments	51.5	103.5
Total financial assets	474.5	429.3
Level 1 \| U.S. government and agencies
Financial assets:
Fixed maturities available-for-sale	96.2	75.7
Short-term investments	51.5	103.5
Level 1 \| States and other territories
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 1 \| Corporate securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Short-term investments	0.0	0.0
Level 1 \| Residential mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 1 \| Commercial mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 1 \| Asset backed securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 2
Financial assets:
Cash, cash equivalents, and restricted cash	0.0	0.0
Fixed maturities available-for-sale	340.1	217.7
Short-term investments	11.3	49.0
Total financial assets	351.4	266.7
Level 2 \| U.S. government and agencies
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Short-term investments	0.0	0.0
Level 2 \| States and other territories
Financial assets:
Fixed maturities available-for-sale	6.8	8.2
Level 2 \| Corporate securities
Financial assets:
Fixed maturities available-for-sale	242.2	146.4
Short-term investments	11.3	49.0
Level 2 \| Residential mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	57.0	37.7
Level 2 \| Commercial mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	2.9	5.0
Level 2 \| Asset backed securities
Financial assets:
Fixed maturities available-for-sale	31.2	20.4
Level 3
Financial assets:
Cash, cash equivalents, and restricted cash	0.0	0.0
Fixed maturities available-for-sale	0.0	0.0
Short-term investments	0.0	0.0
Total financial assets	0.0	0.0
Level 3 \| U.S. government and agencies
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Short-term investments	0.0	0.0
Level 3 \| States and other territories
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 3 \| Corporate securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Short-term investments	0.0	0.0
Level 3 \| Residential mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 3 \| Commercial mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 3 \| Asset backed securities
Financial assets:
Fixed maturities available-for-sale	$ 0.0	$ 0.0