SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	$ 0		$ 0		$ 0
Future policy benefits, losses, claims and loss expenses	0		0		0
Unearned premiums	0		0		0
Other policy claims and benefits payable	0		0		0
Premiums earned	0		0		0
Benefits, claims, losses, and settlement expenses	0		0		0
Amortization of deferred policy acquisition costs	0		0		0
Other operating expenses	0		0		0
Premiums written	0		0		0
Net investment income of non-insurance operations	32.3		23.3		21.9
OneBeacon
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	123.9		123.5		114.5
Future policy benefits, losses, claims and loss expenses	1,000.0		3,358.6		3,295.5
Unearned premiums	573.8		528.0		627.5
Other policy claims and benefits payable	0		0		0
Premiums earned	1,132.0		1,012.2		1,181.1
Net investment income	53.6	[1]	71.4	[1]	96.6	[1]
Benefits, claims, losses, and settlement expenses	650.0		548.3		685.6
Amortization of deferred policy acquisition costs	249.4		221.2		252.1
Other operating expenses	205.2		162.3		196.1
Premiums written	1,179.2		1,062.7		1,167.7
Sirius Group
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	71.4		63.5		61.6
Future policy benefits, losses, claims and loss expenses	2,168.9		2,343.7		2,441.3
Unearned premiums	350.2		319.0		311.2
Other policy claims and benefits payable	0		0		0
Premiums earned	931.6		912.3		847.9
Net investment income	65.0	[1]	89.9	[1]	90.5	[1]
Benefits, claims, losses, and settlement expenses	543.9		626.0		531.0
Amortization of deferred policy acquisition costs	180.8		181.0		167.5
Other operating expenses	116.4		105.8		99.8
Premiums written	947.7		915.7		865.8
HG Global-BAM [Member]
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	0
Future policy benefits, losses, claims and loss expenses	0
Unearned premiums	0
Other policy claims and benefits payable	0
Premiums earned	0
Net investment income	2.2	[1]
Benefits, claims, losses, and settlement expenses	0
Amortization of deferred policy acquisition costs	0
Other operating expenses	0.2
Premiums written	0
Other Operations
SUPPLEMENTARY INSURANCE INFORMATION
Net investment income	$ 0.5	[1]	$ (0.1)	[1]	$ (0.1)	[1]

[1] The amounts shown exclude net investment income relating to non-insurance operations in the other operations segment of $32.3, $23.3 and $21.9 for the twelve months ended December 31, 2012, 2011 and 2010, respectively.