Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Schedule of Investments [Line Items]
Amortized Cost, net	$ 5,875.0	$ 5,652.9
Fair Value	5,573.2	5,235.3
Fixed maturity securities
Schedule of Investments [Line Items]
Amortized Cost, net	5,875.0	5,652.9
Gross Unrealized Gains	87.0	62.9
Gross Unrealized Losses	388.8	480.5
Fair Value	5,573.2	5,235.3
Mortgage-backed securities
Schedule of Investments [Line Items]
Amortized Cost, net	770.3	713.4
Gross Unrealized Gains	9.3	4.4
Gross Unrealized Losses	51.3	64.6
Fair Value	728.3	653.2
Other, including U.S. Treasury securities
Schedule of Investments [Line Items]
Amortized Cost, net	436.1	450.8
Gross Unrealized Gains	1.4	0.8
Gross Unrealized Losses	51.8	62.8
Fair Value	385.7	388.8
Municipal bonds
Schedule of Investments [Line Items]
Amortized Cost, net	1,258.7	1,333.4
Gross Unrealized Gains	29.7	28.6
Gross Unrealized Losses	72.7	91.9
Fair Value	1,215.7	1,270.1
Foreign government bonds
Schedule of Investments [Line Items]
Amortized Cost, net	17.1	23.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.6	1.0
Fair Value	16.5	22.1
Corporate bonds
Schedule of Investments [Line Items]
Amortized Cost, net	2,104.8	1,969.9
Gross Unrealized Gains	32.3	23.1
Gross Unrealized Losses	192.6	220.3
Fair Value	1,944.5	1,772.7
Other asset-backed securities
Schedule of Investments [Line Items]
Amortized Cost, net	1,288.0	1,162.3
Gross Unrealized Gains	14.3	6.0
Gross Unrealized Losses	19.8	39.9
Fair Value	$ 1,282.5	$ 1,128.4