Segment Information (Assets) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment reporting information
Total investments	$ 6,886.3	$ 7,192.6
Reinsurance recoverable on paid and unpaid losses	507.5	453.5
Assets held for sale	58.1	1,880.1
Assets	10,456.9	12,144.3
Loss and loss adjustment expense reserves	3,159.8	3,079.3	3,168.9	5,702.3
Liabilities held for sale	0	1,880.1
Liabilities	5,917.3	7,746.9
Non-controlling interests	543.0	491.9
White Mountains’ common shareholders’ equity	3,996.6	3,905.5
OneBeacon
Segment reporting information
Total investments	2,527.0	2,364.9
Reinsurance recoverable on paid and unpaid losses	173.8	89.9
Assets held for sale	58.1	1,880.1
Assets	3,561.0	5,191.3
Loss and loss adjustment expense reserves	1,342.2	1,054.3
Liabilities held for sale		1,880.1
Liabilities	2,510.5	4,083.9
Non-controlling interests	262.1	276.8
White Mountains’ common shareholders’ equity	788.4	830.6
Sirius Group
Segment reporting information
Total investments	3,178.4	3,251.4
Reinsurance recoverable on paid and unpaid losses	333.6	363.6
Assets held for sale	0	0
Assets	5,034.7	5,124.1
Loss and loss adjustment expense reserves	1,809.8	2,025.0
Liabilities held for sale		0
Liabilities	3,180.6	3,414.5
Non-controlling interests	250.0	250.0
White Mountains’ common shareholders’ equity	1,604.1	1,459.6
HG Global
Segment reporting information
Total investments	121.0	107.9
Reinsurance recoverable on paid and unpaid losses	0	0
Assets held for sale	0	0
Assets	704.4	675.0
Loss and loss adjustment expense reserves	0	0
Liabilities held for sale		0
Liabilities	120.9	68.9
Non-controlling interests	17.9	16.6
White Mountains’ common shareholders’ equity	565.5	589.5
BAM
Segment reporting information
Total investments	454.2	468.6
Reinsurance recoverable on paid and unpaid losses	0	0
Assets held for sale	0	0
Assets	(89.4)	(77.9)
Loss and loss adjustment expense reserves	0	0
Liabilities held for sale		0
Liabilities	32.5	19.7
Non-controlling interests	(121.9)	(97.6)
White Mountains’ common shareholders’ equity	0	0
Surplus notes	503.0
Accrued interest on surplus notes	74.4	58.6
Other Operations
Segment reporting information
Total investments	605.7	999.8
Reinsurance recoverable on paid and unpaid losses	0.1	0
Assets held for sale	0	0
Assets	1,246.2	1,231.8
Loss and loss adjustment expense reserves	7.8	0
Liabilities held for sale		0
Liabilities	72.8	159.9
Non-controlling interests	134.9	46.1
White Mountains’ common shareholders’ equity	$ 1,038.6	$ 1,025.8