Segment Information (Assets) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2012
Segment reporting information
Total investments	$ 3,380.7	$ 2,714.4
Assets held for sale		3,606.4
Assets	3,659.2	6,520.2
Liabilities held for sale		2,569.3
Liabilities	298.4	2,804.2
White Mountains’ common shareholders’ equity	3,492.5	3,582.7
Non-controlling interests	(131.7)	133.3
HG Global BAM
Segment reporting information
Total investments	693.4	629.7
Assets held for sale		0.0
Assets	747.4	677.6
Liabilities held for sale		0.0
Liabilities	394.9	289.1
White Mountains’ common shareholders’ equity	499.8	522.8
Non-controlling interests	(147.3)	(134.3)
BAM
Segment reporting information
Surplus notes	499.0	300.0
Accrued interest on surplus notes	126.0	108.0
MediaAlpha
Segment reporting information
Total investments	0.0	0.0
Assets held for sale		0.0
Assets	96.5	57.6
Liabilities held for sale		0.0
Liabilities	59.8	28.3
White Mountains’ common shareholders’ equity	23.6	17.6
Non-controlling interests	13.1	11.7
Other Operations
Segment reporting information
Total investments	2,687.3	2,084.7
Assets held for sale		0.0
Assets	3,039.9	2,358.8
Liabilities held for sale		0.0
Liabilities	71.6	97.7
White Mountains’ common shareholders’ equity	2,965.8	2,258.1
Non-controlling interests	2.5	3.0
Total Capital Contributed to Subsidiary	727.7	703.0
Eliminations
Segment reporting information
Assets	(227.9)	(180.2)
Liabilities	(227.9)	(180.2)
Discontinued Operations, Held-for-sale
Segment reporting information
Total investments	0.0	0.0
Assets held for sale		3,606.4
Assets	3.3	3,606.4
Liabilities held for sale		2,569.3
Liabilities	0.0	2,569.3
White Mountains’ common shareholders’ equity	3.3	784.2
Non-controlling interests	0.0	252.9
Continuing Operations
Segment reporting information
Non-controlling interests	(131.7)	133.3
HG Global
Segment reporting information
Non-controlling interests	15.9	16.6
Surplus notes			$ 503.0
HG Global \| BAM
Segment reporting information
Surplus notes	499.0	503.0
Preferred Dividends Payable \| HG Global \| Other Operations
Segment reporting information
Accounts Payable and Accrued Liabilities	$ 227.9	$ 180.2

Sum of the carrying values as of the balance sheet date of obligations incurred through that date, including liabilities incurred and payable to vendors for goods and services received, taxes, interest, rent and utilities, compensation costs, payroll taxes and fringe benefits (other than pension and postretirement obligations), contractual rights and obligations, and statutory obligations.

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