Investment Securities (Investment Holdings) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Investment securities
Fixed maturity investments, at fair value	$ 4,854.4	$ 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	435.9	440.4
Gross unrealized gains	0.9	1.0
Gross unrealized losses	(0.1)	(0.1)
Net foreign currency gains (losses)	(1.2)	(1.2)
Fixed maturity investments, at fair value	435.5	440.1
Debt securities issued by corporations
Investment securities
Cost or amortized cost	2,361.2	2,321.4
Gross unrealized gains	75.7	88.3
Gross unrealized losses	(2.6)	(1.6)
Net foreign currency gains (losses)	(24.6)	(23.0)
Fixed maturity investments, at fair value	2,409.7	2,385.1
Municipal obligations
Investment securities
Cost or amortized cost	5.3	5.3
Gross unrealized gains	0	0
Gross unrealized losses	(0.1)	(0.1)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	5.2	5.2
Mortgage-backed and asset-backed securities
Investment securities
Cost or amortized cost	1,698.7	2,081.0
Gross unrealized gains	12.6	25.1
Gross unrealized losses	(1.9)	(1.1)
Net foreign currency gains (losses)	(2.1)	(9.4)
Fixed maturity investments, at fair value	1,707.3	2,095.6
Foreign government, agency and provincial obligations
Investment securities
Cost or amortized cost	516.4	526.6
Gross unrealized gains	5.4	6.9
Gross unrealized losses	(6.3)	(3.0)
Net foreign currency gains (losses)	(9.4)	(8.6)
Fixed maturity investments, at fair value	506.1	521.9
Non-cumulative perpetual preference shares
Investment securities
Cost or amortized cost	79.9	79.9
Gross unrealized gains	7.7	6.7
Gross unrealized losses	0	0
Net foreign currency gains (losses)	(0.1)	(0.2)
Fixed maturity investments, at fair value	87.5	86.4
Common Stock
Investment securities
Cost or amortized cost	876.8	895.2
Gross unrealized gains	201.7	143.4
Gross unrealized losses	(5.2)	(8.8)
Net foreign currency gains (losses)	(0.1)	(0.1)
Convertible fixed maturity investments
Investment securities
Cost or amortized cost	91.2	121.7
Gross unrealized gains	5.9	6.1
Gross unrealized losses	(1.3)	(0.4)
Net foreign currency gains (losses)	(0.2)	0
Other long-term investments.
Investment securities
Cost or amortized cost	254.5	257.2
Gross unrealized gains	57.0	65.9
Gross unrealized losses	(10.3)	(22.8)
Net foreign currency gains (losses)	(6.0)	(6.1)
Fixed Income Investments
Investment securities
Cost or amortized cost	5,097.4	5,454.6
Gross unrealized gains	102.3	128.0
Gross unrealized losses	(11.0)	(5.9)
Net foreign currency gains (losses)	(37.4)	(42.4)
Total fixed maturity investments including assets held for sale	5,151.3	5,534.3
OneBeacon Runoff
Investment securities
Fixed maturity investments reclassified to assets held for sale	$ (296.9)	$ (338.1)