Goodwill and Intangible Assets - Schedule of Definite and Indefinite Life Intangible Assets (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	$ 750.9	$ 716.8
Indefinite-lived Intangible Assets (Excluding Goodwill)	49.4	49.7
Intangible Assets, Gross (Excluding Goodwill), Total	800.3	766.5
Finite-Lived Intangible Assets, Accumulated Amortization	482.0	449.8
Finite-Lived Intangible Assets, Net	268.9	267.0
Intangible Assets, Net (Including Goodwill)	318.3	316.7
Trade Names
Finite-Lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets (Excluding Goodwill)	5.2	5.2
Insurance Licenses
Finite-Lived Intangible Assets [Line Items]
Indefinite-lived Intangible Assets (Excluding Goodwill)	44.2	44.5
Value of Business Acquired
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	237.5	237.5
Finite-Lived Intangible Assets, Accumulated Amortization	223.7	222.1
Finite-Lived Intangible Assets, Net	13.8	15.4
Customer Relationships
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	43.8	43.8
Finite-Lived Intangible Assets, Accumulated Amortization	42.1	41.1
Finite-Lived Intangible Assets, Net	1.7	2.7
Agent Relationships
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	81.6	81.6
Finite-Lived Intangible Assets, Accumulated Amortization	38.2	31.0
Finite-Lived Intangible Assets, Net	43.4	50.6
Trade Names
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	0.0	1.8
Finite-Lived Intangible Assets, Accumulated Amortization	0.0	1.7
Finite-Lived Intangible Assets, Net	0.0	0.1
Internal Use Software
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	388.0	352.1
Finite-Lived Intangible Assets, Accumulated Amortization	178.0	153.9
Finite-Lived Intangible Assets, Net	$ 210.0	$ 198.2