Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Schedule of Investments [Line Items]
Amortized Cost, net	$ 5,785.4	$ 5,652.9
Fair Value	5,342.7	5,235.3
Fixed maturity securities
Schedule of Investments [Line Items]
Amortized Cost, net	5,785.4	5,652.9
Gross Unrealized Gains	56.7	62.9
Gross Unrealized Losses	499.4	480.5
Fair Value	5,342.7	5,235.3
Mortgage-backed securities
Schedule of Investments [Line Items]
Amortized Cost, net	746.5	713.4
Gross Unrealized Gains	2.9	4.4
Gross Unrealized Losses	71.7	64.6
Fair Value	677.7	653.2
Other, including U.S. Treasury securities
Schedule of Investments [Line Items]
Amortized Cost, net	457.6	450.8
Gross Unrealized Gains	0.4	0.8
Gross Unrealized Losses	66.8	62.8
Fair Value	391.2	388.8
Municipal bonds
Schedule of Investments [Line Items]
Amortized Cost, net	1,282.2	1,333.4
Gross Unrealized Gains	22.6	28.6
Gross Unrealized Losses	97.3	91.9
Fair Value	1,207.5	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	1.2	1.0
Fair Value	21.9	22.1
Corporate bonds
Schedule of Investments [Line Items]
Amortized Cost, net	2,041.1	1,969.9
Gross Unrealized Gains	18.5	23.1
Gross Unrealized Losses	229.7	220.3
Fair Value	1,829.9	1,772.7
Other asset-backed securities
Schedule of Investments [Line Items]
Amortized Cost, net	1,234.9	1,162.3
Gross Unrealized Gains	12.3	6.0
Gross Unrealized Losses	32.7	39.9
Fair Value	$ 1,214.5	$ 1,128.4