Investment Securities (Investment Holdings) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investment securities
Cost or amortized cost	$ 5,189.6	$ 6,228.2
Gross unrealized gains	137.4	119.1
Gross unrealized losses	(4.6)	(18.6)
Net foreign currency gains (losses)	(32.7)	5.0
Total fixed maturity investments including assets held for sale	5,289.7	6,333.7
Fixed maturity investments, at fair value	4,912.4	6,221.9
U.S. Government and agency obligations
Investment securities
Cost or amortized cost	393.7	299.4
Gross unrealized gains	1.2	5.3
Gross unrealized losses	(0.1)	(0.1)
Net foreign currency gains (losses)	(0.1)	0.4
Fixed maturity investments, at fair value	394.7	305.0
Debt securities issued by corporations
Investment securities
Cost or amortized cost	2,213.8	2,072.1
Gross unrealized gains	91.6	73.7
Gross unrealized losses	(3.0)	(7.8)
Net foreign currency gains (losses)	(21.1)	(2.9)
Fixed maturity investments, at fair value	2,281.3	2,135.1
Municipal obligations
Investment securities
Cost or amortized cost	3.8	2.7
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	3.8	2.7
Mortgage-backed and asset-backed securities
Investment securities
Cost or amortized cost	2,023.3	3,190.5
Gross unrealized gains	30.3	25.9
Gross unrealized losses	(1.1)	(3.9)
Net foreign currency gains (losses)	(5.1)	10.4
Fixed maturity investments, at fair value	2,047.4	3,222.9
Foreign government, agency and provincial obligations
Investment securities
Cost or amortized cost	475.2	581.2
Gross unrealized gains	8.0	11.0
Gross unrealized losses	(0.4)	(0.1)
Net foreign currency gains (losses)	(6.4)	(2.9)
Fixed maturity investments, at fair value	476.4	589.2
Non-cumulative perpetual preference shares
Investment securities
Cost or amortized cost	79.8	82.3
Gross unrealized gains	6.3	3.2
Gross unrealized losses	0	(6.7)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	86.1	78.8
Common shareholders' equity.
Investment securities
Cost or amortized cost	870.0	691.7
Gross unrealized gains	139.7	72.0
Gross unrealized losses	(4.9)	(8.7)
Net foreign currency gains (losses)	0	0
Convertible fixed maturity investments
Investment securities
Cost or amortized cost	140.2	139.2
Gross unrealized gains	7.6	6.2
Gross unrealized losses	(5.8)	(1.6)
Net foreign currency gains (losses)	0	0
Other long-term investments.
Investment securities
Cost or amortized cost	264.8	274.4
Gross unrealized gains	57.1	55.5
Gross unrealized losses	(8.6)	(25.2)
Net foreign currency gains (losses)	(6.2)	(3.4)
OneBeacon Runoff [Member]
Investment securities
Fixed maturity investments reclassified to assets held for sale		377.3
AutoOne
Investment securities
Fixed maturity investments reclassified to assets held for sale		$ 111.8