SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Millions	Dec. 31, 2018 USD ($)
Summary of investments other than investments in related parties
Cost	$ 2,537.3
Carrying Value	2,542.9
Fair Value	2,542.9
Fixed maturity investments
Summary of investments other than investments in related parties
Cost	1,088.1
Carrying Value	1,077.5
Fair Value	1,077.5
U.S. Government and agency obligations
Summary of investments other than investments in related parties
Cost	154.0
Carrying Value	153.2
Fair Value	153.2
Debt securities issued by corporations
Summary of investments other than investments in related parties
Cost	519.0
Carrying Value	510.5
Fair Value	510.5
Mortgage and asset-backed securities
Summary of investments other than investments in related parties
Cost	136.1
Carrying Value	133.5
Fair Value	133.5
Municipal obligations
Summary of investments other than investments in related parties
Cost	279.0
Carrying Value	280.3
Fair Value	280.3
Short-term investments
Summary of investments other than investments in related parties
Cost	214.2
Carrying Value	214.2
Fair Value	214.2
Exchange traded funds
Summary of investments other than investments in related parties
Cost	681.8
Carrying Value	675.3
Fair Value	675.3
Common equity securities
Summary of investments other than investments in related parties
Cost	904.7
Carrying Value	925.6
Fair Value	925.6
Banks, trust and insurance companies
Summary of investments other than investments in related parties
Cost	11.8
Carrying Value	13.5
Fair Value	13.5
Industrial, miscellaneous and other
Summary of investments other than investments in related parties
Cost	211.1
Carrying Value	236.8
Fair Value	236.8
Other long-term investments
Summary of investments other than investments in related parties
Cost	330.3
Carrying Value	325.6
Fair Value	$ 325.6