Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Variable Interest Entities
Accumulated losses	$ 2,789.7				$ 2,175.6				$ 2,789.7	$ 2,175.6
Noncontrolling interest in VIE	580.2				607.8				580.2	607.8
Noncontrolling interest income (expense) related to VIE	20.3	(11.0)	20.8	11.4	16.6	24.3	10.7	1.4	41.5	53.0	109.5
OneBeacon
Variable Interest Entities
Noncontrolling interest in VIE	287.2				314.9				287.2	314.9
Reciprocals | OneBeacon
Variable Interest Entities
Number of reciprocals capitalized									3
Houston General Insurance
Variable Interest Entities
Total assets related to consolidated VIE's	105.9				118.0				105.9	118.0
Total liabilities related to consolidated VIE's	126.2				136.2				126.2	136.2
Houston General Insurance | OneBeacon
Variable Interest Entities
Surplus notes	23.7								23.7
Accumulated losses	20.3								20.3
Accrued interest on surplus notes	17.0								17.0
Prospector Offshore Fund
Variable Interest Entities
Capital at risk, investment in the fund	65.3								65.3
Total assets related to consolidated VIE's	135.8				145.3				135.8	145.3
Total liabilities related to consolidated VIE's	31.6				39.1				31.6	39.1
Noncontrolling interest in VIE	38.9				40.6				38.9	40.6
Noncontrolling interest income (expense) related to VIE									0.2	(3.2)	(5.0)
Ownership interest in VIE (as a percent)									62.65%
Tuckerman Fund I
Variable Interest Entities
Total assets related to consolidated VIE's	17.6				19.4				17.6	19.4
Total liabilities related to consolidated VIE's	9.9				12.3				9.9	12.3
Noncontrolling interest in VIE	3.5				2.4				3.5	2.4
Noncontrolling interest income (expense) related to VIE									$ 2.2	$ 1.0	$ 0.7
Ownership interest in VIE (as a percent)									94.00%
Companies owned by the Tuckerman Fund I
Variable Interest Entities
Number of small manufacturing companies owned by the Tuckerman Fund I from whom the entity received shares									2