Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	$ 5,842.5	$ 5,652.9
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	5,842.5	5,652.9
Gross Unrealized Gains	48.4	62.9
Gross Unrealized Losses	503.0	480.5
Fair Value	5,387.9	5,235.3
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	827.9	713.4
Gross Unrealized Gains	2.7	4.4
Gross Unrealized Losses	74.8	64.6
Fair Value	755.8	653.2
Other, including U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	426.5	450.8
Gross Unrealized Gains	0.1	0.8
Gross Unrealized Losses	69.0	62.8
Fair Value	357.6	388.8
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	1,239.1	1,333.4
Gross Unrealized Gains	17.5	28.6
Gross Unrealized Losses	105.8	91.9
Fair Value	1,150.8	1,270.1
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	14.1	23.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	1.0	1.0
Fair Value	13.1	22.1
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	1,995.2	1,969.9
Gross Unrealized Gains	17.3	23.1
Gross Unrealized Losses	230.1	220.3
Fair Value	1,782.4	1,772.7
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	1,339.7	1,162.3
Gross Unrealized Gains	10.8	6.0
Gross Unrealized Losses	22.3	39.9
Fair Value	$ 1,328.2	$ 1,128.4