SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
SUPPLEMENTARY INSURANCE INFORMATION
Net investment income	$ 105.0	$ 110.9	$ 153.6
OneBeacon
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	103.2	103.7	123.9
Future policy benefits, losses, claims and loss expenses	1,342.2	1,054.3	1,000.0
Unearned premiums	588.3	544.9	573.8
Other policy claims and benefits payable	0	0	0
Premiums earned	1,177.1	1,120.4	1,132.0
Net investment income	41.7	41.1	53.6
Benefits, claims, losses, and settlement expenses	815.1	622.1	650.0
Amortization of deferred policy acquisition costs	203.3	208.9	249.4
Other operating expenses	179.2	204.8	205.2
Premiums written	1,216.9	1,088.6	1,179.2
Net investment income	41.7	41.1	53.6
Sirius Group
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	69.9	69.3	71.4
Future policy benefits, losses, claims and loss expenses	1,809.8	2,025.0	2,168.9
Unearned premiums	338.6	343.3	350.2
Other policy claims and benefits payable	0	0	0
Premiums earned	873.9	866.4	931.6
Net investment income	41.1	30.3	65.0
Benefits, claims, losses, and settlement expenses	345.3	418.4	543.9
Amortization of deferred policy acquisition costs	193.6	166.5	180.8
Other operating expenses	129.7	126.1	116.4
Premiums written	882.5	876.6	947.7
HG Global-BAM
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	4.0	1.7
Future policy benefits, losses, claims and loss expenses	0	0
Unearned premiums	27.6	13.2
Other policy claims and benefits payable	0	0
Premiums earned	1.8	0.5
Net investment income	7.1	5.7
Benefits, claims, losses, and settlement expenses	0	0
Amortization of deferred policy acquisition costs	2.1	1.5
Other operating expenses	0.4	0.4
Premiums written	16.2	13.6
Other Operations
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	0	0	0
Future policy benefits, losses, claims and loss expenses	7.8	0	0
Unearned premiums	0.8	0	0
Other policy claims and benefits payable	0	0	0
Premiums earned	6.1	0	0
Net investment income	0.2	0.1	0.5
Benefits, claims, losses, and settlement expenses	8.9	0	0
Amortization of deferred policy acquisition costs	0.8	0	0
Other operating expenses	0	0	0
Premiums written	5.9	0	0
Non-insurance [Member] | Operating Segments
SUPPLEMENTARY INSURANCE INFORMATION
Net investment income	$ 14.9	$ 33.7	$ 32.3