Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended								12 Months Ended
	Dec. 31, 2011 company	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured		15.00%								15.00%
Accumulated losses	$ 2,789.7	$ 2,542.7				$ 2,789.7				$ 2,542.7	$ 2,789.7
Noncontrolling interest in VIE	580.2	526.4				580.2				526.4	580.2
Non-controlling interest in consolidated subsidiaries		12.0	30.9	(12.1)	(16.8)	(20.3)	11.0	(20.8)	(11.4)	(14.0)	41.5	53.0
Number of small manufacturing companies owned by the Tuckerman Fund I from whom the entity received shares	2
HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured		15.00%								15.00%
Noncontrolling interest in VIE	0	16.5				0				16.5	0
OneBeacon
Variable Interest Entity [Line Items]
Noncontrolling interest in VIE	287.2	254.2				287.2				254.2	287.2
Reciprocals | OneBeacon
Variable Interest Entity [Line Items]
Number of reciprocals capitalized										3
Houston General Insurance
Variable Interest Entity [Line Items]
Total assets related to consolidated VIE's	105.9	97.7				105.9				97.7	105.9
Total liabilities related to consolidated VIE's	126.2	120.4				126.2				120.4	126.2
Houston General Insurance | OneBeacon
Variable Interest Entity [Line Items]
Surplus notes		23.7								23.7
Accumulated losses		22.7								22.7
Accrued interest on surplus notes		19.7								19.7
Prospector Offshore Fund
Variable Interest Entity [Line Items]
Total assets related to consolidated VIE's	135.8	151.0				135.8				151.0	135.8
Total liabilities related to consolidated VIE's	31.6	51.1				31.6				51.1	31.6
Noncontrolling interest in VIE	38.9	28.9				38.9				28.9	38.9
Non-controlling interest in consolidated subsidiaries										2.6	0.2	(3.2)
Ownership interest in VIE (as a percent)										71.10%
Captial at risk, investment in the fund		71.1								71.1
Tuckerman Fund I
Variable Interest Entity [Line Items]
Total assets related to consolidated VIE's	17.6					17.6					17.6	19.4
Total liabilities related to consolidated VIE's	9.9					9.9					9.9	12.3
Noncontrolling interest in VIE	3.5					3.5					3.5	2.4
Non-controlling interest in consolidated subsidiaries											2.2	1.0
Entities Owned by Tuckerman Fund I
Variable Interest Entity [Line Items]
Number of small manufacturing companies owned by the Tuckerman Fund I from whom the entity received shares											2
Common shareholders’ equity | BAM
Variable Interest Entity [Line Items]
Issuance of notes										503.0
Common shareholders’ equity | Tuckerman Fund I
Variable Interest Entity [Line Items]
Ownership interest in VIE (as a percent)	94.00%										94.00%
HG Global
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured		15.00%								15.00%
Number of Trust		2								2
Surplus notes		503.0								503.0
BAM
Variable Interest Entity [Line Items]
Percentage of par value of policy reinsured		60.00%								60.00%
Right to elect eirectors on board number										2
Surplus notes		$ 0								$ 0