Investment Securities (The Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (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
Trading Securities, Gross Unrealized Loss	4.6	18.6
Net foreign currency gains (losses)	(32.7)	5.0
Fixed maturity investments, at fair value	4,912.4	6,221.9
Common equity securities, at fair value	1,004.8	755.0
Convertible fixed maturity investments, at fair value	142.0	143.8
Other long-term investments	307.1	301.3
Common shareholders' equity.
Investment securities
Cost or amortized cost	870.0	691.7
Gross unrealized gains	139.7	72.0
Trading Securities, Gross Unrealized Loss	4.9	8.7
Net foreign currency gains (losses)	0	0
Common equity securities, at fair value	1,004.8	755.0
Convertible Debt Securities [Member]
Investment securities
Cost or amortized cost	140.2	139.2
Gross unrealized gains	7.6	6.2
Trading Securities, Gross Unrealized Loss	5.8	1.6
Net foreign currency gains (losses)	0	0
Convertible fixed maturity investments, at fair value	142.0	143.8
Other long-term investments.
Investment securities
Cost or amortized cost	264.8	274.4
Gross unrealized gains	57.1	55.5
Trading Securities, Gross Unrealized Loss	8.6	25.2
Net foreign currency gains (losses)	(6.2)	(3.4)
Other long-term investments	307.1	301.3
OneBeacon Runoff [Member]
Investment securities
Fixed maturity investments reclassified to assets held for sale		$ 377.3