Segment Information (Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment reporting information
Total investments	$ 5,334.8	$ 4,271.1
Reinsurance recoverable on paid and unpaid losses	179.5	193.5
Assets held for sale	10.1	4,790.4
Assets	6,544.7	10,282.6
Loss and loss adjustment expense reserves	1,365.6	1,389.8	$ 1,342.2	$ 1,054.3
Liabilities held for sale	5.1	3,047.4
Liabilities	2,807.5	5,914.6
Non-controlling interests	133.9	454.8
White Mountains’ common shareholders’ equity	3,603.3	3,913.2
OneBeacon
Segment reporting information
Total investments	2,620.4	2,591.4
Reinsurance recoverable on paid and unpaid losses	179.5	193.5
Assets held for sale	0.0	0.0
Assets	3,589.4	3,600.8
Loss and loss adjustment expense reserves	1,365.6	1,389.8
Liabilities held for sale	0.0	0.0
Liabilities	2,564.2	2,596.3
Non-controlling interests	244.6	249.3
White Mountains’ common shareholders’ equity	780.6	755.2
HG Global
Segment reporting information
Total investments	161.6	136.2
Reinsurance recoverable on paid and unpaid losses	0.0	0.0
Assets held for sale	0.0	0.0
Assets	787.0	739.0
Loss and loss adjustment expense reserves	0.0	0.0
Liabilities held for sale	0.0	0.0
Liabilities	247.6	181.2
Non-controlling interests	16.6	17.1
White Mountains’ common shareholders’ equity	522.8	540.7
BAM
Segment reporting information
Total investments	468.1	460.3
Reinsurance recoverable on paid and unpaid losses	0.0	0.0
Assets held for sale	0.0	0.0
Assets	(78.9)	(91.1)
Loss and loss adjustment expense reserves	0.0	0.0
Liabilities held for sale	0.0	0.0
Liabilities	72.0	48.9
Non-controlling interests	(150.9)	(140.0)
White Mountains’ common shareholders’ equity	0.0	0.0
Surplus notes	503.0
Accrued interest on surplus notes	108.0	90.2
Other Operations
Segment reporting information
Total investments	2,084.7	1,083.2
Reinsurance recoverable on paid and unpaid losses	0.0	0.0
Assets held for sale	0.0	0.0
Assets	2,417.3	1,378.8
Loss and loss adjustment expense reserves	0.0	0.0
Liabilities held for sale	0.0	0.0
Liabilities	98.8	176.1
Non-controlling interests	23.6	78.4
White Mountains’ common shareholders’ equity	2,294.9	1,124.3
Total Capital Contributed to Subsidiary	703.0	676.0
Consolidation, Eliminations [Member]
Segment reporting information
Assets	(180.2)	(135.3)
Liabilities	(180.2)	(135.3)
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	10.1	4,790.4
Assets	10.1	4,790.4
Loss and loss adjustment expense reserves	0.0	0.0
Liabilities held for sale	5.1	3,047.4
Liabilities	5.1	3,047.4
Non-controlling interests	0.0	0.0
White Mountains’ common shareholders’ equity	5.0	1,493.0
Continuing Operations [Member]
Segment reporting information
Non-controlling interests	133.9	204.8
SIG Preference Shares
Segment reporting information
Non-controlling interests		250.0
HG Global
Segment reporting information
Non-controlling interests	16.6	17.1
Surplus notes	300.0				$ 503.0
Preferred Dividends Payable [Member] | HG Global | Other Operations
Segment reporting information
Accounts Payable and Accrued Liabilities	$ 180.2	$ 135.3