SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Millions	Dec. 31, 2019 USD ($)
Summary of investments other than investments in related parties
Cost	$ 2,599.8
Carrying Value	2,947.2
Fair Value	2,947.2
Fixed maturity investments
Summary of investments other than investments in related parties
Cost	1,177.9
Carrying Value	1,205.8
Fair Value	1,205.8
U.S. Government and agency obligations
Summary of investments other than investments in related parties
Cost	231.7
Carrying Value	232.5
Fair Value	232.5
Debt securities issued by corporations
Summary of investments other than investments in related parties
Cost	454.9
Carrying Value	467.2
Fair Value	467.2
Municipal obligations
Summary of investments other than investments in related parties
Cost	284.7
Carrying Value	297.1
Fair Value	297.1
Mortgage and asset-backed securities
Summary of investments other than investments in related parties
Cost	206.6
Carrying Value	209.0
Fair Value	209.0
Short-term investments
Summary of investments other than investments in related parties
Cost	201.2
Carrying Value	201.2
Fair Value	201.2
Exchange traded funds
Summary of investments other than investments in related parties
Cost	436.0
Carrying Value	536.4
Fair Value	536.4
Common equity securities
Summary of investments other than investments in related parties
Cost	553.3
Carrying Value	683.9
Fair Value	683.9
Other
Summary of investments other than investments in related parties
Cost	117.3
Carrying Value	147.5
Fair Value	147.5
Other long-term investments
Summary of investments other than investments in related parties
Cost	667.4
Carrying Value	856.3
Fair Value	$ 856.3