Investment Securities (Details 4) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Investment securities
Cost or amortized cost	$ 5,130.9	$ 6,228.2
Gross unrealized gains	115.1	119.1
Gross unrealized losses	(5.5)	(18.6)
Net foreign currency gains (losses)	8.2	5.0
Total fixed maturity investments including assets held for sale		6,333.7
Fixed maturity investments reclassified to assets held for sale		(111.8)
Fixed maturity investments, at fair value	5,248.7	6,221.9
Carrying value, common equity securities	919.2	755.0
Carrying value, convertible fixed securities	145.5	143.8
Carrying value, other long-term investments	292.5	301.3
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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	260.4	299.4
Gross unrealized gains	1.1	5.3
Gross unrealized losses	(0.1)	(0.1)
Net foreign currency gains (losses)	0.5	0.4
Fixed maturity investments, at fair value	261.9	305.0
Debt securities issued by corporations
Investment securities
Cost or amortized cost	2,259.1	2,072.1
Gross unrealized gains	82.9	73.7
Gross unrealized losses	(3.5)	(7.8)
Net foreign currency gains (losses)	2.1	(2.9)
Fixed maturity investments, at fair value	2,340.6	2,135.1
Municipal obligations
Investment securities
Cost or amortized cost	3.8	2.7
Gross unrealized gains	0.1	0
Gross unrealized losses	0	0
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	3.9	2.7
Mortgage-backed and asset-backed securities
Investment securities
Cost or amortized cost	1,982.8	3,190.5
Gross unrealized gains	19.0	25.9
Gross unrealized losses	(1.8)	(3.9)
Net foreign currency gains (losses)	9.0	10.4
Fixed maturity investments, at fair value	2,009.0	3,222.9
Foreign government, agency and provincial obligations
Investment securities
Cost or amortized cost	542.5	581.2
Gross unrealized gains	7.5	11.0
Gross unrealized losses	(0.1)	(0.1)
Net foreign currency gains (losses)	(3.4)	(2.9)
Fixed maturity investments, at fair value	546.5	589.2
Non-cumulative perpetual preference shares
Investment securities
Cost or amortized cost	82.3	82.3
Gross unrealized gains	4.5	3.2
Gross unrealized losses	0	(6.7)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	86.8	78.8
Common shareholders' equity.
Investment securities
Cost or amortized cost	849.5	691.7
Gross unrealized gains	87.2	72.0
Gross unrealized losses	(17.5)	(8.7)
Net foreign currency gains (losses)	0	0
Carrying value, common equity securities	919.2	755.0
Convertible fixed maturity investments
Investment securities
Cost or amortized cost	143.1	139.2
Gross unrealized gains	4.8	6.2
Gross unrealized losses	(2.4)	(1.6)
Net foreign currency gains (losses)	0	0
Carrying value, convertible fixed securities	145.5	143.8
Other long-term investments.
Investment securities
Cost or amortized cost	248.0	274.4
Gross unrealized gains	56.4	55.5
Gross unrealized losses	(9.5)	(25.2)
Net foreign currency gains (losses)	(2.4)	(3.4)
Carrying value, other long-term investments	$ 292.5	$ 301.3