Investments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Fixed Maturity Investments and Equity Securities
Cost or Amortized Cost, Fixed maturity investments	$ 2,464.6	$ 2,421.9
Cost or Amortized Cost, Equity securities	148.0	106.6
Fair Value, Fixed maturity investments	2,496.3	2,430.8
Fair Value, Equity securities	156.9	117.3
Net unrealized losses associated with Montpeliers open short positions	2.5	2.4
Average duration of fixed maturities	6 months 26 days
Fixed maturity investments rated "A" (Strong) or better by Standard & Poor's (as a percent)	69.90%
Fixed maturity investments rated "BBB" (Good) by Standard & Poor's (as a percent)	10.40%
Fixed maturity investments unrated or rated below "BBB" by Standard & Poor's (as a percent)	19.70%
Fixed maturity investments
Fixed Maturity Investments and Equity Securities
Open short positions	208.4	130.0
Equity and investment option and future positions
Fixed Maturity Investments and Equity Securities
Open short positions	12.0	25.6
Corporate debt securities
Fixed Maturity Investments and Equity Securities
Cost or Amortized Cost, Fixed maturity investments	913.2	886.7
Fair Value, Fixed maturity investments	929.7	906.8
Debt securities issued by non-U.S. governments and their agencies
Fixed Maturity Investments and Equity Securities
Cost or Amortized Cost, Fixed maturity investments	459.4	109.6
Fair Value, Fixed maturity investments	465.4	108.2
Debt securities issued/sponsored by the U.S. Treasury and its agencies
Fixed Maturity Investments and Equity Securities
Cost or Amortized Cost, Fixed maturity investments	389.1	533.0
Fair Value, Fixed maturity investments	388.5	524.5
Residential mortgage-backed securities
Fixed Maturity Investments and Equity Securities
Cost or Amortized Cost, Fixed maturity investments	291.7	430.3
Fair Value, Fixed maturity investments	294.4	423.5
Commercial mortgage-backed securities
Fixed Maturity Investments and Equity Securities
Cost or Amortized Cost, Fixed maturity investments	134.1	137.7
Fair Value, Fixed maturity investments	135.1	138.8
Debt securities issued by U.S. states and political subdivisions
Fixed Maturity Investments and Equity Securities
Cost or Amortized Cost, Fixed maturity investments	39.3	47.2
Fair Value, Fixed maturity investments	42.2	47.1
Other debt obligations
Fixed Maturity Investments and Equity Securities
Cost or Amortized Cost, Fixed maturity investments	237.8	277.4
Fair Value, Fixed maturity investments	241.0	281.9
Exchange-listed funds
Fixed Maturity Investments and Equity Securities
Cost or Amortized Cost, Equity securities	136.1	84.5
Fair Value, Equity securities	145.5	95.2
Other
Fixed Maturity Investments and Equity Securities
Cost or Amortized Cost, Equity securities	11.9	22.1
Fair Value, Equity securities	$ 11.4	$ 22.1