Investment Securities (Investment Holdings) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Investment securities
Fixed maturity investments, at fair value	$ 4,914.0	$ 5,196.2
Fixed maturity investments reclassified to assets held for sale		(338.1)
U.S. Government and agency obligations
Investment securities
Cost or amortized cost	390.8	440.4
Trading Securities, Unrealized Holding Gain	0.6	1.0
Trading Securities, Unrealized Holding Loss	(0.8)	(0.1)
Net foreign currency gains (losses)	(2.4)	(1.2)
Fixed maturity investments, at fair value	388.2	440.1
Debt securities issued by corporations
Investment securities
Cost or amortized cost	2,303.1	2,321.4
Trading Securities, Unrealized Holding Gain	48.5	88.3
Trading Securities, Unrealized Holding Loss	(12.7)	(1.6)
Net foreign currency gains (losses)	(21.7)	(23.0)
Fixed maturity investments, at fair value	2,317.2	2,385.1
Municipal obligations
Investment securities
Cost or amortized cost	5.3	5.3
Trading Securities, Unrealized Holding Gain	0.5	0
Trading Securities, Unrealized Holding Loss	(0.3)	(0.1)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	5.5	5.2
Mortgage-backed and asset-backed securities
Investment securities
Cost or amortized cost	1,931.2	2,081.0
Trading Securities, Unrealized Holding Gain	2.8	25.1
Trading Securities, Unrealized Holding Loss	(12.4)	(1.1)
Net foreign currency gains (losses)	(5.8)	(9.4)
Fixed maturity investments, at fair value	1,915.8	2,095.6
Foreign government, agency and provincial obligations
Investment securities
Cost or amortized cost	452.7	526.6
Trading Securities, Unrealized Holding Gain	4.0	6.9
Trading Securities, Unrealized Holding Loss	(7.6)	(3.0)
Net foreign currency gains (losses)	(6.3)	(8.6)
Fixed maturity investments, at fair value	442.8	521.9
Preferred Stock
Investment securities
Cost or amortized cost	79.9	79.9
Trading Securities, Unrealized Holding Gain	4.5	6.7
Trading Securities, Unrealized Holding Loss	0	0
Net foreign currency gains (losses)	(0.2)	(0.2)
Fixed maturity investments, at fair value	84.2	86.4
Common Stock
Investment securities
Cost or amortized cost	881.5	895.2
Trading Securities, Unrealized Holding Gain	233.8	143.4
Trading Securities, Unrealized Holding Loss	(3.5)	(8.8)
Net foreign currency gains (losses)	(1.1)	(0.1)
Fixed maturity investments, at fair value	1,110.7	1,029.7
Convertible fixed maturity investments
Investment securities
Cost or amortized cost	79.7	121.7
Trading Securities, Unrealized Holding Gain	5.8	6.1
Trading Securities, Unrealized Holding Loss	(0.6)	(0.4)
Net foreign currency gains (losses)	(0.1)	0
Fixed maturity investments, at fair value	84.8	127.4
Other long-term investments.
Investment securities
Cost or amortized cost	252.6	257.2
Trading Securities, Unrealized Holding Gain	77.8	65.9
Trading Securities, Unrealized Holding Loss	(27.4)	(22.8)
Net foreign currency gains (losses)	(3.0)	(6.1)
Fixed maturity investments, at fair value	300.0	294.2
Fixed Income Investments
Investment securities
Cost or amortized cost	5,163.0	5,454.6
Trading Securities, Unrealized Holding Gain	60.9	128.0
Trading Securities, Unrealized Holding Loss	(33.8)	(5.9)
Net foreign currency gains (losses)	(36.4)	(42.4)
Total fixed maturity investments including assets held for sale	5,153.7	5,534.3
OneBeacon Runoff
Investment securities
Fixed maturity investments reclassified to assets held for sale	$ (239.7)	$ (338.1)