Investments - Continuous Loss Position (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Less than 12 months
Fair Value	$ 82.8	$ 96.2
Gross Unrealized Losses	(0.1)	(0.8)
12 months or more
Fair Value	42.2	51.1
Gross Unrealized Losses	(2.2)	(2.7)
Total
Fair Value	125.1	147.3
Gross Unrealized Losses	(2.3)	(3.5)
U.S. government and agencies
Less than 12 months
Fair Value	8.9	14.5
Gross Unrealized Losses	0.0	(0.1)
12 months or more
Fair Value	1.9	1.9
Gross Unrealized Losses	(0.1)	(0.1)
Total
Fair Value	10.8	16.4
Gross Unrealized Losses	(0.1)	(0.2)
States and other territories
Less than 12 months
Fair Value	1.9	5.0
Gross Unrealized Losses	0.0	(0.1)
12 months or more
Fair Value	4.8	5.5
Gross Unrealized Losses	(0.2)	(0.2)
Total
Fair Value	6.7	10.5
Gross Unrealized Losses	(0.2)	(0.3)
Corporate securities
Less than 12 months
Fair Value	36.4	55.2
Gross Unrealized Losses	(0.1)	(0.5)
12 months or more
Fair Value	19.1	25.4
Gross Unrealized Losses	(0.4)	(0.6)
Total
Fair Value	55.6	80.6
Gross Unrealized Losses	(0.5)	(1.1)
Residential mortgage-backed securities
Less than 12 months
Fair Value	3.8	7.4
Gross Unrealized Losses	0.0	(0.1)
12 months or more
Fair Value	9.9	10.1
Gross Unrealized Losses	(1.2)	(1.4)
Total
Fair Value	13.7	17.5
Gross Unrealized Losses	(1.2)	(1.5)
Commercial mortgage-backed securities
Less than 12 months
Fair Value	1.4	0.4
Gross Unrealized Losses	0.0	0.0
12 months or more
Fair Value	4.1	4.1
Gross Unrealized Losses	(0.3)	(0.4)
Total
Fair Value	5.5	4.5
Gross Unrealized Losses	(0.3)	(0.4)
Asset backed securities
Less than 12 months
Fair Value	2.1	0.0
Gross Unrealized Losses	0.0	0.0
12 months or more
Fair Value	2.4	4.1
Gross Unrealized Losses	0.0	0.0
Total
Fair Value	4.5	4.1
Gross Unrealized Losses	0.0	0.0
U.S. government and agencies
Less than 12 months
Fair Value	1.9	0.0
Gross Unrealized Losses	0.0	0.0
12 months or more
Fair Value	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Total
Fair Value	1.9	0.0
Gross Unrealized Losses	0.0	0.0
Commercial paper
Less than 12 months
Fair Value	8.4	6.3
Gross Unrealized Losses	0.0	0.0
12 months or more
Fair Value	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Total
Fair Value	8.4	6.3
Gross Unrealized Losses	0.0	0.0
Corporate securities
Less than 12 months
Fair Value	18.0	7.4
Gross Unrealized Losses	0.0	0.0
12 months or more
Fair Value	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Total
Fair Value	18.0	7.4
Gross Unrealized Losses	$ 0.0	$ 0.0

Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, 12 Months or Longer

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Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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