Investments - Continuous Loss Position (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Less than 12 months
Fair Value	$ 100.8	$ 200.6
Gross Unrealized Losses	(0.5)	0.0
12 months or more
Fair Value	47.6	81.8
Gross Unrealized Losses	(3.6)	(4.1)
Total
Fair Value	148.4	282.4
Gross Unrealized Losses	(4.1)	(4.1)
U.S. government and agencies
Less than 12 months
Fair Value	9.0	4.3
Gross Unrealized Losses	(0.1)	0.0
12 months or more
Fair Value	5.9	10.5
Gross Unrealized Losses	(0.1)	(0.2)
Total
Fair Value	14.9	14.8
Gross Unrealized Losses	(0.2)	(0.2)
States and other territories
Less than 12 months
Fair Value	0.2	1.5
Gross Unrealized Losses	0.0	0.0
12 months or more
Fair Value	7.5	7.4
Gross Unrealized Losses	(0.4)	(0.4)
Total
Fair Value	7.7	8.9
Gross Unrealized Losses	(0.4)	(0.4)
Corporate securities
Less than 12 months
Fair Value	46.9	5.7
Gross Unrealized Losses	(0.3)	0.0
12 months or more
Fair Value	5.9	37.4
Gross Unrealized Losses	(1.1)	(1.3)
Total
Fair Value	52.8	43.1
Gross Unrealized Losses	(1.4)	(1.3)
Foreign securities
Less than 12 months
Fair Value	0.0	0.0
Gross Unrealized Losses	0.0	0.0
12 months or more
Fair Value	4.0	0.9
Gross Unrealized Losses	0.0	0.0
Total
Fair Value	4.0	0.9
Gross Unrealized Losses	0.0	0.0
Residential mortgage-backed securities
Less than 12 months
Fair Value	4.7	0.0
Gross Unrealized Losses	(0.1)	0.0
12 months or more
Fair Value	10.7	11.6
Gross Unrealized Losses	(1.5)	(1.3)
Total
Fair Value	15.4	11.6
Gross Unrealized Losses	(1.6)	(1.3)
Commercial mortgage-backed securities
Less than 12 months
Fair Value	0.4	0.4
Gross Unrealized Losses	0.0	0.0
12 months or more
Fair Value	5.9	5.8
Gross Unrealized Losses	(0.4)	(0.6)
Total
Fair Value	6.3	6.2
Gross Unrealized Losses	(0.4)	(0.6)
Asset backed securities
Less than 12 months
Fair Value	1.8	1.6
Gross Unrealized Losses	0.0	0.0
12 months or more
Fair Value	7.7	8.2
Gross Unrealized Losses	(0.1)	(0.3)
Total
Fair Value	9.5	9.8
Gross Unrealized Losses	(0.1)	(0.3)
U.S. government and agencies
Less than 12 months
Fair Value	5.2	137.7
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	5.2	137.7
Gross Unrealized Losses	0.0	0.0
Commercial paper
Less than 12 months
Fair Value	19.4	34.5
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	19.4	34.5
Gross Unrealized Losses	0.0	0.0
Corporate securities
Less than 12 months
Fair Value	13.2	14.9
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	13.2	14.9
Gross Unrealized Losses	$ 0.0	$ 0.0

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

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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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