Goodwill and Intangible Assets - Finite-lived Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	$ 164.0	$ 158.6	$ 142.2
Agency relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	159.2	159.2	159.2
Accumulated Amortization	91.0	88.1	79.6
Net Carrying Amount	68.2	71.1	79.6
Software rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	69.1	69.1	69.1
Accumulated Amortization	69.1	67.0	60.5
Net Carrying Amount	0.0	2.1	8.6
Trade name
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	3.6	3.6	3.6
Accumulated Amortization	3.3	2.9	1.5
Net Carrying Amount	0.3	0.7	2.1
Finite-Lived Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	231.9	231.9	231.9
Accumulated Amortization	163.4	158.0	141.6
Net Carrying Amount	$ 68.5	$ 73.9	$ 90.3