FAIR VALUE OF FINANCIAL INSTRUMENTS - Balance Sheet Grouping (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Assets
AFS securities	$ 128.8	$ 129.9
Cash equivalents	487.3	127.0
Total assets at fair value	616.1	256.9
U.S. Treasury securities and agencies
Assets
AFS securities	11.0	23.3
Municipal securities
Assets
AFS securities	20.2	20.6
Corporate debt securities
Assets
AFS securities	57.8	48.7
Residential mortgage-backed securities
Assets
AFS securities	5.2	3.5
Commercial mortgage-backed securities
Assets
AFS securities	23.2	30.3
Other debt obligations
Assets
AFS securities	11.0	3.5
Total fixed maturities
Assets
AFS securities	128.4	129.9
Short-term investments
Assets
AFS securities	0.4
Level 1
Assets
Cash equivalents	487.3	127.0
Total assets at fair value	496.5	149.6
Level 1 | U.S. Treasury securities and agencies
Assets
AFS securities	9.2	22.6
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	9.2	22.6
Level 1 | Short-term investments
Assets
AFS securities	0.0
Level 2
Assets
Cash equivalents	0.0	0.0
Total assets at fair value	119.6	107.3
Level 2 | U.S. Treasury securities and agencies
Assets
AFS securities	1.8	0.7
Level 2 | Municipal securities
Assets
AFS securities	20.2	20.6
Level 2 | Corporate debt securities
Assets
AFS securities	57.8	48.7
Level 2 | Residential mortgage-backed securities
Assets
AFS securities	5.2	3.5
Level 2 | Commercial mortgage-backed securities
Assets
AFS securities	23.2	30.3
Level 2 | Other debt obligations
Assets
AFS securities	11.0	3.5
Level 2 | Total fixed maturities
Assets
AFS securities	119.2	107.3
Level 2 | Short-term investments
Assets
AFS securities	0.4
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