Segment Information (Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment reporting information
Total investments	$ 4,280.7	$ 3,742.4
Reinsurance recoverable on paid and unpaid losses	194.0	173.9
Assets held for sale	4,407.0	4,630.6
Assets	10,284.5	10,455.7
Loss and loss adjustment expense reserves	1,395.8	1,350.0	$ 1,054.3	$ 1,000.0
Liabilities held for sale	2,884.0	3,105.3
Liabilities	5,916.5	5,917.3
Non-controlling interests	454.8	542.7
White Mountains’ common shareholders’ equity	3,913.2	3,995.7
OneBeacon
Segment reporting information
Total investments	2,591.4	2,525.8
Reinsurance recoverable on paid and unpaid losses	193.5	173.8
Assets held for sale	0.0	58.1
Assets	3,602.7	3,559.8
Loss and loss adjustment expense reserves	1,389.8	1,342.2
Liabilities held for sale	0.0	0.0
Liabilities	2,598.2	2,510.5
Non-controlling interests	249.3	261.8
White Mountains’ common shareholders’ equity	755.2	787.5
HG Global
Segment reporting information
Total investments	136.2	121.0
Reinsurance recoverable on paid and unpaid losses	0.0	0.0
Assets held for sale	0.0	0.0
Assets	739.0	704.4
Loss and loss adjustment expense reserves	0.0	0.0
Liabilities held for sale	0.0	0.0
Liabilities	181.2	121.0
Non-controlling interests	17.1	17.9
White Mountains’ common shareholders’ equity	540.7	565.5
BAM
Segment reporting information
Total investments	460.3	454.2
Reinsurance recoverable on paid and unpaid losses	0.0	0.0
Assets held for sale	0.0	0.0
Assets	(91.1)	(89.4)
Loss and loss adjustment expense reserves	0.0	0.0
Liabilities held for sale	0.0	0.0
Liabilities	48.9	32.5
Non-controlling interests	(140.0)	(121.9)
White Mountains’ common shareholders’ equity	0.0	0.0
Surplus notes	503.0
Accrued interest on surplus notes	90.2	74.4
Other Operations
Segment reporting information
Total investments	1,092.8	641.4
Reinsurance recoverable on paid and unpaid losses	0.5	0.1
Assets held for sale	0.0	0.0
Assets	1,626.9	1,708.4
Loss and loss adjustment expense reserves	6.0	7.8
Liabilities held for sale	0.0	0.0
Liabilities	204.2	148.0
Non-controlling interests	78.4	134.9
White Mountains’ common shareholders’ equity	1,344.3	1,425.5
Discontinued Operations, Held-for-sale [Member]
Segment reporting information
Total investments	0.0	0.0
Reinsurance recoverable on paid and unpaid losses	0.0	0.0
Assets held for sale	4,407.0	4,572.5
Assets	4,407.0	4,572.5
Loss and loss adjustment expense reserves	0.0	0.0
Liabilities held for sale	2,884.0	3,105.3
Liabilities	2,884.0	3,105.3
Non-controlling interests	0.0	0.0
White Mountains’ common shareholders’ equity	1,273.0	1,217.2
Continuing Operations [Member]
Segment reporting information
Non-controlling interests	$ 204.8	292.7
SIG Preference Shares
Segment reporting information
Non-controlling interests		$ 250.0