Investments - Fixed Maturities Securities and Short-term Investments (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 148.5	$ 127.3
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	(7.1)	(6.4)
Fair Value	141.5	121.1
Amortized Cost	188.6	325.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.1)	(0.8)
Fair Value	188.5	324.8
Amortized Cost	337.1	452.9
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	(7.2)	(7.2)
Total investments	330.0	445.9
U.S. government and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22.5	21.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.4)	(0.5)
Fair Value	22.1	21.1
Amortized Cost	123.4	129.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.1)	(0.2)
Fair Value	123.3	128.9
States and other territories
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10.1	8.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.6)	(0.6)
Fair Value	9.5	8.3
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	71.9	54.8
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	(2.6)	(2.4)
Fair Value	69.4	52.5
Amortized Cost	17.6	49.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	17.6	49.4
Foreign securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	0.9	0.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.1)
Fair Value	0.9	0.8
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	21.2	20.4
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	(2.3)	(1.6)
Fair Value	18.9	18.9
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7.7	6.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.7)	(0.7)
Fair Value	7.0	5.8
Asset backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	14.2	14.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.5)	(0.5)
Fair Value	13.7	13.7
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	47.6	147.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.6)
Fair Value	$ 47.6	$ 146.5

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