Investment Securities (Details 4) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Investment securities
Cost or amortized cost	$ 6,228.2	$ 5,755.9
Gross unrealized gains	119.1	133.6
Gross unrealized losses	(18.6)	(44.2)
Net foreign currency gains (losses)	5.0	(59.2)
Total fixed maturity investments including assets held for sale	6,333.7
Fixed maturity investments, at fair value	6,221.9	5,786.1
Cost or amortized cost
Total	6,228.2	5,755.9
Carrying value
Carrying value, common equity securities	755.0	667.0
Carrying value, convertible fixed securities	143.8	143.0
Carrying value, other long-term investments	301.3	372.1
Sales and maturities of investments, excluding short-term investments	5,044.5	4,492.6	3,748.5
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Investment securities
Fixed maturity investments reclassified to assets held for sale	(111.8)
U.S. Government and agency obligations
Investment securities
Cost or amortized cost	299.4	385.9
Gross unrealized gains	5.3	13.6
Gross unrealized losses	(0.1)	(0.8)
Net foreign currency gains (losses)	0.4
Fixed maturity investments, at fair value	305.0	398.7
Cost or amortized cost
Total	299.4	385.9
Debt securities issued by corporations
Investment securities
Cost or amortized cost	2,072.1	2,149.2
Gross unrealized gains	73.7	87.9
Gross unrealized losses	(7.8)	(21.1)
Net foreign currency gains (losses)	(2.9)	(37.9)
Fixed maturity investments, at fair value	2,135.1	2,178.1
Cost or amortized cost
Total	2,072.1	2,149.2
Municipal obligations
Investment securities
Cost or amortized cost	2.7	3.3
Gross unrealized gains		0.1
Gross unrealized losses		(0.1)
Fixed maturity investments, at fair value	2.7	3.3
Cost or amortized cost
Total	2.7	3.3
Mortgage-backed and asset-backed securities
Investment securities
Cost or amortized cost	3,190.5	2,082.0
Gross unrealized gains	25.9	18.2
Gross unrealized losses	(3.9)	(15.6)
Net foreign currency gains (losses)	10.4	(12.9)
Fixed maturity investments, at fair value	3,222.9	2,071.7
Cost or amortized cost
Securities not having single maturity date	3,190.5
Total	3,190.5	2,082.0
Carrying value
Securities not having single maturity date	3,222.9
Foreign government, agency and provincial obligations
Investment securities
Cost or amortized cost	581.2	1,053.6
Gross unrealized gains	11.0	7.7
Gross unrealized losses	(0.1)	(6.6)
Net foreign currency gains (losses)	(2.9)	(8.3)
Fixed maturity investments, at fair value	589.2	1,046.4
Cost or amortized cost
Total	581.2	1,053.6
Non-cumulative perpetual preference shares
Investment securities
Cost or amortized cost	82.3	81.9
Gross unrealized gains	3.2	6.1
Gross unrealized losses	(6.7)
Net foreign currency gains (losses)		(0.1)
Fixed maturity investments, at fair value	78.8	87.9
Cost or amortized cost
Securities not having single maturity date	82.3
Total	82.3	81.9
Carrying value
Securities not having single maturity date	78.8
Fixed maturity investments and convertible fixed maturity investments, excluding mortgage-backed securities, asset-backed securities and preferred stock
Cost or amortized cost
Due in one year or less	669.6
Due after one year through five years	1,809.1
Due after five years through ten years	563.1
Due after ten years	52.8
Carrying value
Due in one year or less	681.0
Due after one year through five years	1,857.5
Due after five years through ten years	582.8
Due after ten years	54.5
Fixed maturity investments and convertible fixed maturity investments
Investment securities
Cost or amortized cost	6,367.4
Cost or amortized cost
Total	6,367.4
Carrying value
Total	6,477.5
Common shareholders' equity.
Investment securities
Cost or amortized cost	691.7	561.2
Gross unrealized gains	72.0	105.5
Gross unrealized losses	(8.7)	(1.5)
Net foreign currency gains (losses)		1.8
Cost or amortized cost
Total	691.7	561.2
Convertible fixed maturities
Investment securities
Cost or amortized cost	139.2	126.9
Gross unrealized gains	6.2	16.2
Gross unrealized losses	(1.6)	(0.1)
Cost or amortized cost
Total	139.2	126.9
Other long-term investments.
Investment securities
Cost or amortized cost	274.4	329.5
Gross unrealized gains	55.5	62.4
Gross unrealized losses	(25.2)	(13.0)
Net foreign currency gains (losses)	(3.4)	(6.8)
Cost or amortized cost
Total	$ 274.4	$ 329.5