SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
SUPPLEMENTARY INSURANCE INFORMATION
Net investment income	$ 60.8	$ 59.5	$ 59.8
OneBeacon
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	100.7	103.2	103.7
Future policy benefits, losses, claims and loss expenses	1,389.8	1,342.2	1,054.3
Unearned premiums	560.3	588.3	544.9
Other policy claims and benefits payable	0.0	0.0	0.0
Premiums earned	1,176.2	1,177.1	1,120.4
Net investment income	45.9	43.4	43.0
Benefits, claims, losses, and settlement expenses	700.7	815.1	622.1
Amortization of deferred policy acquisition costs	213.8	203.3	208.9
Other operating expenses	218.2	179.2	204.8
Premiums written	1,136.6	1,216.9	1,088.6
Net investment income	45.9	43.4	43.0
HG Global-BAM
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	6.9	4.0
Future policy benefits, losses, claims and loss expenses	0.0	0.0
Unearned premiums	50.2	27.6
Other policy claims and benefits payable	0.0	0.0
Premiums earned	3.3	1.8
Net investment income	6.1	7.1
Benefits, claims, losses, and settlement expenses	0.0	0.0
Amortization of deferred policy acquisition costs	2.9	2.1
Other operating expenses	0.4	0.4
Premiums written	25.9	16.2
Other Operations
SUPPLEMENTARY INSURANCE INFORMATION
Deferred acquisition costs	0.0	0.0	0.0
Future policy benefits, losses, claims and loss expenses	6.0	7.8	0.0
Unearned premiums	2.1	0.8	0.0
Other policy claims and benefits payable	0.0	0.0	0.0
Premiums earned	8.7	6.1	0.0
Net investment income	0.2	0.2	0.1
Benefits, claims, losses, and settlement expenses	8.2	8.9	0.0
Amortization of deferred policy acquisition costs	3.4	0.8	0.0
Other operating expenses	0.0	0.0	0.0
Premiums written	10.1	5.9	0.0
Non-insurance [Member] | Operating Segments
SUPPLEMENTARY INSURANCE INFORMATION
Net investment income	$ 8.6	$ 8.8	$ 11.0