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Includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Entity may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Compensating balance arrangements that do not legally restrict the withdrawal or usage of cash amounts may be reported as Cash and Cash Equivalents, while legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or company statements of intention with regard to particular deposits are not generally reported as cash and cash equivalents. Includes cash and cash equivalents associated with the entity's continuing operations. Excludes cash and cash equivalents associated with the disposal group (and discontinued operation).

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 3) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains' common shareholders	$ 800.2	$ (15.8)	$ 11.7	$ (28.2)	$ 72.9	$ 50.7	$ 2.5	$ (39.6)	$ 767.9	$ 86.5	$ 470.0
Charges (credits) to reconcile net income to net cash (used for) provided from operations:
Net realized and unrealized investment (gains) losses									(122.8)	(82.6)	(361.7)
Net federal income tax (payments) receipts									12.5	(47.0)	(17.5)
Net change in other assets and liabilities, net									(96.5)	(238.6)	27.5
Net cash (used for) provided from operations									(114.5)	56.0	(46.7)
Cash flows from investing activities:
Net decrease in short-term investments									174.5	475.6	218.4
Net change in unsettled investment purchases and sales									47.0	1.6	52.1
Net cash provided from investing activities									799.6	485.3	365.1
Cash flows from financing activities:
Repayment of debt										(14.0)	(242.8)
Proceeds from issuances of common shares									0.9	0.7	0.9
Repurchases and retirement of common shares									(253.0)	(225.6)	(0.4)
Dividends paid on common shares									(8.0)	(8.8)	(8.9)
Net cash (used for) provided from financing activities									(460.2)	(550.2)	(360.2)
Net increase (decrease) in cash during year									223.4	(0.8)	(35.0)
Cash balance at beginning of year	251.9				103.6				251.9	103.6	119.9	158.7
Cash balance at end of year	251.9				103.6				251.9	103.6	119.9	158.7
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income attributable to White Mountains' common shareholders									767.9	86.5	470.0
Charges (credits) to reconcile net income to net cash (used for) provided from operations:
Net realized and unrealized investment (gains) losses									(3.1)	0.1	1.7
Undistributed current (earnings) losses from subsidiaries									(806.7)	(128.2)	(511.9)
Dividends received from subsidiaries											8.4
Net change in other assets and liabilities, net									13.7	27.9	18.6
Net cash (used for) provided from operations									(28.2)	(13.7)	(13.2)
Cash flows from investing activities:
Net decrease in short-term investments									267.3	166.5	20.8
Purchases of investment securities									(237.3)	(5.8)	(28.4)
Sales and maturities of investment securities									59.0
Issuance of debt (to) from subsidiaries										86.8
Repayment of debt (to) from subsidiaries									192.5		200.0
Distributions from subsidiaries									7.2		29.0
Net cash provided from investing activities									288.7	247.5	221.4
Cash flows from financing activities:
Repayment of debt											(200.0)
Proceeds from issuances of common shares									0.9	0.7	0.9
Repurchases and retirement of common shares									(253.0)	(225.6)	(0.4)
Dividends paid on common shares									(8.0)	(8.8)	(8.9)
Net cash (used for) provided from financing activities									(260.1)	(233.7)	(208.4)
Net increase (decrease) in cash during year									0.4	0.1	(0.2)
Cash balance at beginning of year	0.7				0.3				0.7	0.3	0.2	0.4
Cash balance at end of year	$ 0.7				$ 0.3				$ 0.7	$ 0.3	$ 0.2	$ 0.4