SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Summary of investments other than investments in related parties
Cost	$ 3,958.8
Carrying Value	4,262.2
Fair Value	4,262.2
Fair value investments	4,050.9	$ 3,343.6	$ 3,889.0
Fixed maturity investments
Summary of investments other than investments in related parties
Cost	2,639.1
Carrying Value	2,639.7
Fair Value	2,639.7
US Government and agency obligations
Summary of investments other than investments in related parties
Cost	160.4
Carrying Value	160.0
Fair Value	160.0
Debt securities issued by corporations
Summary of investments other than investments in related parties
Cost	1,001.0
Carrying Value	1,000.0
Fair Value	1,000.0
Municipal obligations
Summary of investments other than investments in related parties
Cost	227.8
Carrying Value	228.8
Fair Value	228.8
Mortgage-backed and asset-backed securities
Summary of investments other than investments in related parties
Cost	1,170.6
Carrying Value	1,167.0
Fair Value	1,167.0
Foreign government, agency and provincial obligations
Summary of investments other than investments in related parties
Cost	1.0
Carrying Value	1.2
Fair Value	1.2
Redeemable preferred stocks
Summary of investments other than investments in related parties
Cost	78.3
Carrying Value	82.7
Fair Value	82.7
Short-term investments
Summary of investments other than investments in related parties
Cost	211.3
Carrying Value	211.3
Fair Value	211.3
Exchange Traded Funds [Member]
Summary of investments other than investments in related parties
Cost	147.2
Carrying Value	141.8
Fair Value	141.8
Common equity securities
Summary of investments other than investments in related parties
Cost	822.5
Carrying Value	1,113.9
Fair Value	1,113.9
Banks, trust and insurance companies
Summary of investments other than investments in related parties
Cost	436.9
Carrying Value	694.7
Fair Value	694.7
Public utilities
Summary of investments other than investments in related parties
Cost	0.0
Carrying Value	0.0
Fair Value	0.0
Industrial, miscellaneous and other
Summary of investments other than investments in related parties
Cost	238.4
Carrying Value	277.4
Fair Value	277.4
Other long-term investments
Summary of investments other than investments in related parties
Cost	285.9
Carrying Value	297.3
Fair Value	297.3
Fair value measured on a recurring basis
Summary of investments other than investments in related parties
Fair value investments	4,262.2	3,720.4
Fair value measured on a recurring basis | Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	297.3	309.9
Level 3 | Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	297.3	304.2	189.2
Level 3 | Fair value measured on a recurring basis
Summary of investments other than investments in related parties
Fair value investments	367.3	420.1
Level 3 | Fair value measured on a recurring basis | Other long-term investments
Summary of investments other than investments in related parties
Carrying value of investment accounted for using the equity method	3.8	5.2	$ 6.8
Fair value investments	297.3	304.2
Affordable Housing Development Fund [Member] | Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	$ 14.7	$ 16.8