SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Millions	Dec. 31, 2021 USD ($)
Summary of investments other than investments in related parties
Cost	$ 3,784.4
Carrying Value	4,265.3
Fair Value	4,265.3
Fixed maturity investments
Summary of investments other than investments in related parties
Cost	1,895.5
Carrying Value	1,908.9
Fair Value	1,908.9
U.S. Government and agency obligations
Summary of investments other than investments in related parties
Cost	212.1
Carrying Value	211.5
Fair Value	211.5
Debt securities issued by corporations
Summary of investments other than investments in related parties
Cost	993.3
Carrying Value	992.9
Fair Value	992.9
Municipal obligations
Summary of investments other than investments in related parties
Cost	276.4
Carrying Value	291.9
Fair Value	291.9
Mortgage and asset-backed securities
Summary of investments other than investments in related parties
Cost	277.2
Carrying Value	277.6
Fair Value	277.6
Collateralized loan obligations
Summary of investments other than investments in related parties
Cost	136.5
Carrying Value	135.0
Fair Value	135.0
Short-term investments
Summary of investments other than investments in related parties
Cost	465.9
Carrying Value	465.9
Fair Value	465.9
Investment in MediaAlpha
Summary of investments other than investments in related parties
Cost	0.0
Carrying Value	261.6
Fair Value	261.6
Common equity securities - Industrial, Miscellaneous, and Other
Summary of investments other than investments in related parties
Cost	236.3
Carrying Value	251.1
Fair Value	251.1
Other long-term investments
Summary of investments other than investments in related parties
Cost	1,186.7
Carrying Value	1,377.8
Fair Value	$ 1,377.8