Fair Value Measurement - Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Financial assets:
Cash, cash equivalents, and restricted cash	$ 269.4	$ 244.5	$ 392.9	$ 818.7
Fixed maturities available-for-sale	141.5	121.1
Short-term investments	188.5	324.8
Total financial assets	599.4	690.4
Financial liabilities:
Contingent consideration liability	13.5	11.9
Total financial liabilities	13.7	12.2
Public warrants
Financial liabilities:
Warrants liabilities	0.1	0.2
Private placement warrants
Financial liabilities:
Warrants liabilities	0.1	0.1
U.S. government and agencies
Financial assets:
Fixed maturities available-for-sale	22.1	21.1
Short-term investments	123.3	128.9
States and other territories
Financial assets:
Fixed maturities available-for-sale	9.5	8.3
Corporate securities
Financial assets:
Fixed maturities available-for-sale	69.4	52.5
Short-term investments	17.6	49.4
Foreign securities
Financial assets:
Fixed maturities available-for-sale	0.9	0.8
Residential mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	18.9	18.9
Commercial mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	7.0	5.8
Asset backed securities
Financial assets:
Fixed maturities available-for-sale	13.7	13.7
Commercial paper
Financial assets:
Short-term investments	47.6	146.5
Level 1
Financial assets:
Cash, cash equivalents, and restricted cash	269.4	244.5
Fixed maturities available-for-sale	22.1	21.1
Short-term investments	123.3	128.9
Total financial assets	414.8	394.5
Financial liabilities:
Contingent consideration liability	0.0	0.0
Total financial liabilities	0.1	0.2
Level 1 | Public warrants
Financial liabilities:
Warrants liabilities	0.1	0.2
Level 1 | Private placement warrants
Financial liabilities:
Warrants liabilities	0.0	0.0
Level 1 | U.S. government and agencies
Financial assets:
Fixed maturities available-for-sale	22.1	21.1
Short-term investments	123.3	128.9
Level 1 | States and other territories
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 1 | Corporate securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Short-term investments	0.0	0.0
Level 1 | Foreign securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 1 | Residential mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 1 | Commercial mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 1 | Asset backed securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 1 | Commercial paper
Financial assets:
Short-term investments	0.0	0.0
Level 2
Financial assets:
Cash, cash equivalents, and restricted cash	0.0	0.0
Fixed maturities available-for-sale	119.4	100.0
Short-term investments	65.2	195.9
Total financial assets	184.6	295.9
Financial liabilities:
Contingent consideration liability	0.0	0.0
Total financial liabilities	0.1	0.1
Level 2 | Public warrants
Financial liabilities:
Warrants liabilities	0.0	0.0
Level 2 | Private placement warrants
Financial liabilities:
Warrants liabilities	0.1	0.1
Level 2 | U.S. government and agencies
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Short-term investments	0.0	0.0
Level 2 | States and other territories
Financial assets:
Fixed maturities available-for-sale	9.5	8.3
Level 2 | Corporate securities
Financial assets:
Fixed maturities available-for-sale	69.4	52.5
Short-term investments	17.6	49.4
Level 2 | Foreign securities
Financial assets:
Fixed maturities available-for-sale	0.9	0.8
Level 2 | Residential mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	18.9	18.9
Level 2 | Commercial mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	7.0	5.8
Level 2 | Asset backed securities
Financial assets:
Fixed maturities available-for-sale	13.7	13.7
Level 2 | Commercial paper
Financial assets:
Short-term investments	47.6	146.5
Level 3
Financial assets:
Cash, cash equivalents, and restricted cash	0.0	0.0
Fixed maturities available-for-sale	0.0	0.0
Short-term investments	0.0	0.0
Total financial assets	0.0	0.0
Financial liabilities:
Contingent consideration liability	13.5	11.9
Total financial liabilities	13.5	11.9
Level 3 | Public warrants
Financial liabilities:
Warrants liabilities	0.0	0.0
Level 3 | Private placement warrants
Financial liabilities:
Warrants liabilities	0.0	0.0
Level 3 | U.S. government and agencies
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Short-term investments	0.0	0.0
Level 3 | States and other territories
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 3 | Corporate securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Short-term investments	0.0	0.0
Level 3 | Foreign securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 3 | Residential mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 3 | Commercial mortgage-backed securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 3 | Asset backed securities
Financial assets:
Fixed maturities available-for-sale	0.0	0.0
Level 3 | Commercial paper
Financial assets:
Short-term investments	$ 0.0	$ 0.0