SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Summary of investments other than investments in related parties
Cost	$ 8,179.5
Carrying Value	8,379.8
Fair Value	8,379.8
AutoOne
Summary of investments other than investments in related parties
Investments reclassified to assets held for sale	111.8
Fixed maturity investments.
Summary of investments other than investments in related parties
Cost	6,228.2
Carrying Value	6,333.7
Fair Value	6,333.7
U.S. Government and agency obligations
Summary of investments other than investments in related parties
Cost	299.4
Carrying Value	305.0
Fair Value	305.0
Debt securities issued by corporations
Summary of investments other than investments in related parties
Cost	2,072.1
Carrying Value	2,135.1
Fair Value	2,135.1
Mortgage-backed and asset-backed securities
Summary of investments other than investments in related parties
Cost	3,190.5
Carrying Value	3,222.9
Fair Value	3,222.9
Municipal obligations
Summary of investments other than investments in related parties
Cost	2.7
Carrying Value	2.7
Fair Value	2.7
Foreign government, agency and provincial obligations
Summary of investments other than investments in related parties
Cost	581.2
Carrying Value	589.2
Fair Value	589.2
Non-cumulative perpetual preference shares
Summary of investments other than investments in related parties
Cost	82.3
Carrying Value	78.8
Fair Value	78.8
Short-term investments
Summary of investments other than investments in related parties
Cost	846.0
Carrying Value	846.0
Fair Value	846.0
Common equity securities
Summary of investments other than investments in related parties
Cost	691.7
Carrying Value	755.0
Fair Value	755.0
Banks, trust and insurance companies
Summary of investments other than investments in related parties
Cost	205.3
Carrying Value	219.2
Fair Value	219.2
Public utilities
Summary of investments other than investments in related parties
Cost	39.4
Carrying Value	42.0
Fair Value	42.0
Industrial, miscellaneous and other
Summary of investments other than investments in related parties
Cost	447.0
Carrying Value	493.8
Fair Value	493.8
Convertible fixed maturities
Summary of investments other than investments in related parties
Cost	139.2
Carrying Value	143.8
Fair Value	143.8
Other long-term investments
Summary of investments other than investments in related parties
Cost	274.4
Carrying Value	301.3
Fair Value	$ 301.3