Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Schedule of Investments [Line Items]
Amortized Cost, net	$ 5,816.5	$ 5,652.9
Fair Value	5,345.4	5,235.3
Fixed maturity securities
Schedule of Investments [Line Items]
Amortized Cost, net	5,816.5	5,652.9
Gross Unrealized Gains	48.2	62.9
Gross Unrealized Losses	519.3	480.5
Fair Value	5,345.4	5,235.3
Mortgage-backed securities
Schedule of Investments [Line Items]
Amortized Cost, net	742.0	713.4
Gross Unrealized Gains	2.6	4.4
Gross Unrealized Losses	75.4	64.6
Fair Value	669.2	653.2
Other, including U.S. Treasury securities
Schedule of Investments [Line Items]
Amortized Cost, net	441.3	450.8
Gross Unrealized Gains	0.1	0.8
Gross Unrealized Losses	67.9	62.8
Fair Value	373.5	388.8
Municipal bonds
Schedule of Investments [Line Items]
Amortized Cost, net	1,259.3	1,333.4
Gross Unrealized Gains	19.5	28.6
Gross Unrealized Losses	102.1	91.9
Fair Value	1,176.7	1,270.1
Foreign government bonds
Schedule of Investments [Line Items]
Amortized Cost, net	23.1	23.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	1.1	1.0
Fair Value	22.0	22.1
Corporate bonds
Schedule of Investments [Line Items]
Amortized Cost, net	2,073.4	1,969.9
Gross Unrealized Gains	14.9	23.1
Gross Unrealized Losses	242.1	220.3
Fair Value	1,846.2	1,772.7
Other asset-backed securities
Schedule of Investments [Line Items]
Amortized Cost, net	1,277.4	1,162.3
Gross Unrealized Gains	11.1	6.0
Gross Unrealized Losses	30.7	39.9
Fair Value	$ 1,257.8	$ 1,128.4