Investment Securities (Details 4) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Investment securities
Cost or amortized cost	$ 5,513.3	$ 5,755.9
Gross unrealized gains	113.7	133.6
Gross unrealized losses	(14.1)	(44.2)
Net foreign currency gains (losses)	(54.0)	(59.2)
Fixed maturity investments, at fair value	5,558.9	5,786.1
Carrying value, common equity securities	703.8	667.0
Carrying value, convertible fixed securities	119.7	143.0
Carrying value, other long-term investments	333.3	372.1
U.S. Government and agency obligations
Investment securities
Cost or amortized cost	290.6	385.9
Gross unrealized gains	6.1	13.6
Gross unrealized losses		(0.8)
Net foreign currency gains (losses)	(0.5)
Fixed maturity investments, at fair value	296.2	398.7
Debt securities issued by corporations
Investment securities
Cost or amortized cost	2,001.9	2,149.2
Gross unrealized gains	76.4	87.9
Gross unrealized losses	(8.9)	(21.1)
Net foreign currency gains (losses)	(36.6)	(37.9)
Fixed maturity investments, at fair value	2,032.8	2,178.1
Municipal obligations
Investment securities
Cost or amortized cost	3.3	3.3
Gross unrealized gains		0.1
Gross unrealized losses		(0.1)
Fixed maturity investments, at fair value	3.3	3.3
Mortgage-backed and asset-backed securities
Investment securities
Cost or amortized cost	2,306.9	2,082.0
Gross unrealized gains	21.1	18.2
Gross unrealized losses	(4.8)	(15.6)
Net foreign currency gains (losses)	(15.2)	(12.9)
Fixed maturity investments, at fair value	2,308.0	2,071.7
Foreign government, agency and provincial obligations
Investment securities
Cost or amortized cost	829.6	1,053.6
Gross unrealized gains	3.8	7.7
Gross unrealized losses	(0.4)	(6.6)
Net foreign currency gains (losses)	(1.5)	(8.3)
Fixed maturity investments, at fair value	831.5	1,046.4
Non-cumulative perpetual preference shares
Investment securities
Cost or amortized cost	81.0	81.9
Gross unrealized gains	6.3	6.1
Net foreign currency gains (losses)	(0.2)	(0.1)
Fixed maturity investments, at fair value	87.1	87.9
Common shareholders' equity
Investment securities
Cost or amortized cost	593.0	561.2
Gross unrealized gains	115.7	105.5
Gross unrealized losses	(5.8)	(1.5)
Net foreign currency gains (losses)	0.9	1.8
Carrying value, common equity securities	703.8	667.0
Convertible fixed maturity investments
Investment securities
Cost or amortized cost	111.7	126.9
Gross unrealized gains	9.7	16.2
Gross unrealized losses	(1.7)	(0.1)
Carrying value, convertible fixed securities	119.7	143.0
Other long-term investments
Investment securities
Cost or amortized cost	301.9	329.5
Gross unrealized gains	55.8	62.4
Gross unrealized losses	(13.7)	(13.0)
Net foreign currency gains (losses)	(10.7)	(6.8)
Carrying value, other long-term investments	$ 333.3	$ 372.1