Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	$ 5,652.9	$ 5,756.9
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	5,652.9	5,756.9
Gross Unrealized Gains	62.9	35.0
Gross Unrealized Losses	480.5	606.9
Fair Value	5,235.3	5,185.0
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	713.4	638.2
Gross Unrealized Gains	4.4	1.3
Gross Unrealized Losses	64.6	69.1
Fair Value	653.2	570.4
Other, including U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	450.8	410.0
Gross Unrealized Gains	0.8	0.5
Gross Unrealized Losses	62.8	67.8
Fair Value	388.8	342.7
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	1,333.4	1,380.9
Gross Unrealized Gains	28.6	16.9
Gross Unrealized Losses	91.9	128.1
Fair Value	1,270.1	1,269.7
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	23.1	35.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	1.0	1.6
Fair Value	22.1	33.5
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	1,969.9	2,161.2
Gross Unrealized Gains	23.1	12.7
Gross Unrealized Losses	220.3	272.2
Fair Value	1,772.7	1,901.7
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, net	1,162.3	1,131.5
Gross Unrealized Gains	6.0	3.6
Gross Unrealized Losses	39.9	68.1
Fair Value	$ 1,128.4	$ 1,067.0