SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of investments other than investments in related parties
Fixed maturity investments, at fair value	$ 4,250.2	$ 2,630.2
Cost	5,308.8
Carrying Value	5,341.5
Fair Value	5,341.5
Fair value investments	5,039.7	4,050.9	$ 3,343.6
Fixed maturity investments
Summary of investments other than investments in related parties
Cost	4,266.0
Carrying Value	4,256.8
Fair Value	4,256.8
US Government and agency obligations
Summary of investments other than investments in related parties
Cost	281.7
Carrying Value	278.3
Fair Value	278.3
Debt securities issued by corporations
Summary of investments other than investments in related parties
Cost	1,512.6
Carrying Value	1,509.4
Fair Value	1,509.4
Municipal obligations
Summary of investments other than investments in related parties
Cost	308.8
Carrying Value	309.0
Fair Value	309.0
Mortgage and asset-backed securities
Summary of investments other than investments in related parties
Cost	2,141.7
Carrying Value	2,132.9
Fair Value	2,132.9
Foreign government, agency and provincial obligations
Summary of investments other than investments in related parties
Cost	12.9
Carrying Value	13.2
Fair Value	13.2
Preferred stocks
Summary of investments other than investments in related parties
Cost	8.3
Carrying Value	14.0
Fair Value	14.0
Short-term investments
Summary of investments other than investments in related parties
Cost	287.1
Carrying Value	287.1
Fair Value	287.1
Exchange Traded Funds [Member]
Summary of investments other than investments in related parties
Cost	316.4
Carrying Value	321.6
Fair Value	321.6
Common equity securities
Summary of investments other than investments in related parties
Cost	440.8
Carrying Value	474.3
Fair Value	474.3
Banks, trust and insurance companies
Summary of investments other than investments in related parties
Cost	11.2
Carrying Value	11.6
Fair Value	11.6
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	113.2
Carrying Value	141.1
Fair Value	141.1
Other long-term investments
Summary of investments other than investments in related parties
Cost	314.9
Carrying Value	323.3
Fair Value	323.3
Fair value measured on a recurring basis
Summary of investments other than investments in related parties
Fair value investments	5,195.9	4,134.4
Fair value measured on a recurring basis | Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	177.7	169.5
Level 3 | Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	177.7	169.5	125.9
Level 3 | Fair value measured on a recurring basis
Summary of investments other than investments in related parties
Fair value investments	177.7	239.5
Level 3 | Fair value measured on a recurring basis | Forward Contracts [Member]
Summary of investments other than investments in related parties
Fair value investments	(1.2)
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	3.5	3.8	5.2
Fair value investments	177.7	169.5
Affordable Housing Development Fund [Member] | Other long-term investments
Summary of investments other than investments in related parties
Fair value investments	12.3	14.7	16.8
Star & Shield Insurance Exchange [Member] | Fixed maturity investments
Summary of investments other than investments in related parties
Fixed maturity investments, at fair value	6.6	9.5	$ 10.1
Star & Shield Insurance Exchange [Member] | Short-term investments
Summary of investments other than investments in related parties
Fixed maturity investments, at fair value	$ 0.1	$ 0.1