Investment Securities (Investment Holdings Fixed Maturity Table) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,785.5	$ 5,030.5
Fixed maturity investments reclassified to assets held for sale		(236.3)
U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	254.4	365.5
Trading Securities, Unrealized Holding Gain	0.2	0.5
Trading Securities, Unrealized Holding Loss	(0.5)	(1.0)
Net foreign currency gains (losses)	5.6	(2.5)
Fixed maturity investments, at fair value	259.7	362.5
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	2,314.6	2,330.7
Trading Securities, Unrealized Holding Gain	48.9	44.0
Trading Securities, Unrealized Holding Loss	(6.3)	(13.2)
Net foreign currency gains (losses)	30.1	(14.3)
Fixed maturity investments, at fair value	2,387.3	2,347.2
Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	78.9	18.3
Trading Securities, Unrealized Holding Gain	1.3	0
Trading Securities, Unrealized Holding Loss	(0.3)	(0.4)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	79.9	17.9
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	1,846.7	2,027.3
Trading Securities, Unrealized Holding Gain	6.8	2.4
Trading Securities, Unrealized Holding Loss	(4.6)	(9.9)
Net foreign currency gains (losses)	16.0	(5.3)
Fixed maturity investments, at fair value	1,864.9	2,014.5
Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	311.6	444.2
Trading Securities, Unrealized Holding Gain	3.7	3.7
Trading Securities, Unrealized Holding Loss	(0.8)	(3.2)
Net foreign currency gains (losses)	(3.3)	(4.8)
Fixed maturity investments, at fair value	311.2	439.9
Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	79.7	79.9
Trading Securities, Unrealized Holding Gain	6.7	5.1
Trading Securities, Unrealized Holding Loss	0	0
Net foreign currency gains (losses)	0	(0.2)
Fixed maturity investments, at fair value	86.4	84.8
Fixed Income Investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	4,885.9	5,265.9
Trading Securities, Unrealized Holding Gain	67.6	55.7
Trading Securities, Unrealized Holding Loss	(12.5)	(27.7)
Net foreign currency gains (losses)	48.4	(27.1)
Total fixed maturity investments including assets held for sale	4,989.4	5,266.8
OneBeacon Runoff
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments reclassified to assets held for sale	$ (203.9)	$ (236.3)