FAIR VALUE OF FINANCIAL INSTRUMENTS - Balance Sheet Grouping (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Assets
AFS securities	$ 274.8	$ 166.8
Cash equivalents	416.6	439.6
Total assets at fair value	691.4	606.4
U.S. Treasury securities and agencies
Assets
AFS securities	34.3	14.6
Municipal securities
Assets
AFS securities	23.3	24.1
Corporate debt securities
Assets
AFS securities	105.0	65.2
Residential mortgage-backed securities
Assets
AFS securities	20.9	11.9
Commercial mortgage-backed securities
Assets
AFS securities	52.3	29.8
Other debt obligations
Assets
AFS securities	30.4	20.3
Total fixed maturities
Assets
AFS securities	266.2	165.9
Short-term investments
Assets
AFS securities	8.6	0.9
Level 1
Assets
Cash equivalents	416.6	439.6
Total assets at fair value	458.6	453.8
Level 1 | U.S. Treasury securities and agencies
Assets
AFS securities	33.6	13.3
Level 1 | Municipal securities
Assets
AFS securities	0.0	0.0
Level 1 | Corporate debt securities
Assets
AFS securities	0.0	0.0
Level 1 | Residential mortgage-backed securities
Assets
AFS securities	0.0	0.0
Level 1 | Commercial mortgage-backed securities
Assets
AFS securities	0.0	0.0
Level 1 | Other debt obligations
Assets
AFS securities	0.0	0.0
Level 1 | Total fixed maturities
Assets
AFS securities	33.6	13.3
Level 1 | Short-term investments
Assets
AFS securities	8.4	0.9
Level 2
Assets
Cash equivalents	0.0	0.0
Total assets at fair value	232.8	152.6
Level 2 | U.S. Treasury securities and agencies
Assets
AFS securities	0.7	1.3
Level 2 | Municipal securities
Assets
AFS securities	23.3	24.1
Level 2 | Corporate debt securities
Assets
AFS securities	105.0	65.2
Level 2 | Residential mortgage-backed securities
Assets
AFS securities	20.9	11.9
Level 2 | Commercial mortgage-backed securities
Assets
AFS securities	52.3	29.8
Level 2 | Other debt obligations
Assets
AFS securities	30.4	20.3
Level 2 | Total fixed maturities
Assets
AFS securities	232.6	152.6
Level 2 | Short-term investments
Assets
AFS securities	0.2	0.0
Level 3
Assets
Cash equivalents	0.0	0.0
Total assets at fair value	0.0	0.0
Level 3 | U.S. Treasury securities and agencies
Assets
AFS securities	0.0	0.0
Level 3 | Municipal securities
Assets
AFS securities	0.0	0.0
Level 3 | Corporate debt securities
Assets
AFS securities	0.0	0.0
Level 3 | Residential mortgage-backed securities
Assets
AFS securities	0.0	0.0
Level 3 | Commercial mortgage-backed securities
Assets
AFS securities	0.0	0.0
Level 3 | Other debt obligations
Assets
AFS securities	0.0	0.0
Level 3 | Total fixed maturities
Assets
AFS securities	0.0	0.0
Level 3 | Short-term investments
Assets
AFS securities	$ 0.0	$ 0.0