SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Millions	Dec. 31, 2022 USD ($)
Summary of investments other than investments in related parties
Cost	$ 5,002.6
Carrying Value	5,170.0
Fair Value	5,170.0
Fixed maturity investments
Summary of investments other than investments in related parties
Cost	2,076.0
Carrying Value	1,920.9
Fair Value	1,920.9
U.S. Government and agency obligations
Summary of investments other than investments in related parties
Cost	216.6
Carrying Value	206.4
Fair Value	206.4
Debt securities issued by corporations
Summary of investments other than investments in related parties
Cost	1,098.3
Carrying Value	1,018.8
Fair Value	1,018.8
Municipal obligations
Summary of investments other than investments in related parties
Cost	281.6
Carrying Value	258.6
Fair Value	258.6
Mortgage and asset-backed securities
Summary of investments other than investments in related parties
Cost	288.7
Carrying Value	254.2
Fair Value	254.2
Collateralized loan obligations
Summary of investments other than investments in related parties
Cost	190.8
Carrying Value	182.9
Fair Value	182.9
Short-term investments
Summary of investments other than investments in related parties
Cost	925.2
Carrying Value	924.1
Fair Value	924.1
Investment in MediaAlpha
Summary of investments other than investments in related parties
Cost	0.0
Carrying Value	168.6
Fair Value	168.6
Common equity securities - Industrial, Miscellaneous, and Other
Summary of investments other than investments in related parties
Cost	324.3
Carrying Value	334.6
Fair Value	334.6
Exchange traded funds
Summary of investments other than investments in related parties
Cost	336.3
Carrying Value	333.8
Fair Value	333.8
Total common equity securities
Summary of investments other than investments in related parties
Cost	660.6
Carrying Value	668.4
Fair Value	668.4
Other long-term investments
Summary of investments other than investments in related parties
Cost	1,340.8
Carrying Value	1,488.0
Fair Value	$ 1,488.0