Investment Securities (Investment Holdings) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Investment securities
Fixed maturity investments, at fair value	$ 5,030.5		$ 5,196.2
Including Short Term Fixed
Investment securities
Duration of Fixed Maturities			2 years 1 month 6 days
US Government and agency obligations
Investment securities
Trading Securities, Cost	365.5		440.4
Net foreign currency losses	(2.5)		(1.2)
Fixed maturity investments, at fair value	362.5		440.1
Trading Securities, Unrealized Holding Gain	0.5		1.0
Trading Securities, Unrealized Holding Loss	(1.0)		(0.1)
Debt securities issued by corporations
Investment securities
Trading Securities, Cost	2,330.7		2,321.4
Net foreign currency losses	(14.3)		(23.0)
Fixed maturity investments, at fair value	2,347.2		2,385.1
Trading Securities, Unrealized Holding Gain	44.0		88.3
Trading Securities, Unrealized Holding Loss	(13.2)		(1.6)
Municipal obligations
Investment securities
Trading Securities, Cost	18.3		5.3
Net foreign currency losses	0		0
Fixed maturity investments, at fair value	17.9		5.2
Trading Securities, Unrealized Holding Gain	0		0
Trading Securities, Unrealized Holding Loss	(0.4)		(0.1)
Mortgage-backed and asset-backed securities
Investment securities
Trading Securities, Cost	2,027.3		2,081.0
Net foreign currency losses	(5.3)		(9.4)
Fixed maturity investments, at fair value	2,014.5		2,095.6
Trading Securities, Unrealized Holding Gain	2.4		25.1
Trading Securities, Unrealized Holding Loss	(9.9)		(1.1)
Foreign government, agency and provincial obligations
Investment securities
Trading Securities, Cost	444.2		526.6
Net foreign currency losses	(4.8)		(8.6)
Fixed maturity investments, at fair value	439.9		521.9
Trading Securities, Unrealized Holding Gain	3.7		6.9
Trading Securities, Unrealized Holding Loss	(3.2)		(3.0)
Preferred stocks
Investment securities
Trading Securities, Cost	79.9		79.9
Net foreign currency losses	(0.2)		(0.2)
Fixed maturity investments, at fair value	84.8		86.4
Trading Securities, Unrealized Holding Gain	5.1		6.7
Trading Securities, Unrealized Holding Loss	0		0
Fixed Income Investments [Member]
Investment securities
Trading Securities, Cost	5,265.9		5,454.6
Net foreign currency losses	(27.1)		(42.4)
Total fixed maturity investments including assets held for sale	5,266.8		5,534.3
Trading Securities, Unrealized Holding Gain	55.7		128.0
Trading Securities, Unrealized Holding Loss	(27.7)		(5.9)
Excluding Short Term Investments
Investment securities
Duration of Fixed Maturities			2 years 4 months 24 days
OneBeacon Runoff
Investment securities
Fixed maturity investments reclassified to assets held for sale (1)	$ (236.3)	[1]	$ (338.1)	[1]

[1] Assets held for sale related to discontinued operations. See Note 21.