SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Cash Flows) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Common shareholders’ equity	$ 3,360.8				$ 3,716.0				$ 3,360.8	$ 3,716.0
Net income attributable to White Mountains’s common shareholders (3)	22.5	$ 562.1	$ 13.6	$ 29.0	(38.9)	$ 90.8	$ 340.4	$ 9.5	627.2	401.8	$ 295.2
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses									(133.3)	27.4	(260.5)
Net gain from discontinued operations, net of tax	4.3	$ 539.1	$ 2.8	31.3	8.0	$ 84.4	$ 383.6	47.4	20.5	108.3	116.9
Net change in other assets and liabilities, net									(20.8)	(146.1)	29.0
Net cash provided from (used for) operations									94.6	(149.3)	182.4
Cash flows from investing activities:
Net change in short-term investments									(1.7)	(27.2)	6.9
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									715.5	1,035.4	111.8
Cash Provided by (Used in) Investing Activities, Discontinued Operations									3.0	241.4	(100.5)
Net cash provided from investing activities									718.5	1,276.8	11.3
Payments for (Proceeds from) Other Investing Activities									14.7	(4.8)	(8.8)
Cash flows from financing activities:
Draw down of debt and revolving line of credit									376.0	352.5	171.5
Repayment of revolving line of credit									(365.0)	(404.6)	(76.1)
Proceeds from issuances of common shares									0.0	3.7	0.0
Repurchases and retirement of common shares									(714.6)	(881.3)	(268.6)
Dividends paid on common shares									(4.6)	(5.4)	(6.0)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations									(637.1)	(854.6)	(120.6)
Net cash used for financing activities — discontinued operations (Note 19)									(61.9)	(93.8)	(100.7)
Net cash used for financing activities									(699.0)	(948.4)	(221.3)
Net increase in cash during the year									16.0	7.9	(74.8)
Cash balance at beginning of year				80.2				72.0	80.2	72.0	145.7
Cash balance at end of year	97.1				80.2				97.1	80.2	72.0	$ 145.7
Interest paid									(1.4)	(2.1)	(0.1)
Amortization of restricted share and option awards									14.8	18.5	14.9
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									(62.4)	(172.9)	(66.0)
Cash Provided by (Used in) Operating Activities, Discontinued Operations									157.0	23.6	248.4
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Common shareholders’ equity	3,492.5				3,582.7				3,492.5	3,582.7
Net income attributable to White Mountains’s common shareholders (3)									627.2	401.8	295.2
Charges (credits) to reconcile net income to net cash provided from (used for) operations:
Net realized and unrealized investment (gains) losses									(18.5)	1.1	0.0
Undistributed earnings from subsidiaries (3)									(701.0)	(471.5)	(350.1)
Other non-cash reconciling items primarily amortization of restricted share and option awards (4)									31.1	17.9	(0.4)
Net cash transferred from subsidiary									1,256.7	0.0	0.0
Net change in other assets and liabilities, net									(4.9)	(5.6)	28.1
Net cash provided from (used for) operations									1,190.6	(56.3)	(27.2)
Cash flows from investing activities:
Net change in short-term investments									(24.7)	10.9	7.6
Purchases of investment securities									(474.7)	0.0	0.0
Sales and maturities of investment securities									367.1	0.0	0.0
Issuance of debt from subsidiaries									382.0	992.0	271.0
Repayment of debt to subsidiaries									0.0	(5.0)	(35.0)
Distributions from subsidiaries									0.0	0.0	15.0
Net cash provided from investing activities									(450.3)	997.9	258.6
Cash flows from financing activities:
Draw down of debt and revolving line of credit									350.0	350.0	125.0
Repayment of revolving line of credit									(350.0)	(400.0)	(75.0)
Proceeds from issuances of common shares									0.0	3.7	0.0
Repurchases and retirement of common shares									(714.6)	(881.3)	(268.6)
Dividends paid on common shares									(4.6)	(5.4)	(6.0)
Payments of restricted shares withholding taxes									(9.3)	(5.8)	(6.7)
Net cash used for financing activities									(728.5)	(939.0)	(231.3)
Net increase in cash during the year									11.8	2.8	0.1
Cash balance at beginning of year				$ 3.1				$ 0.3	3.1	0.3	0.2
Cash balance at end of year	14.9				3.1				14.9	3.1	0.3	0.2
Interest paid									(0.6)	(1.2)	0.0
Equity in earnings from consolidated and unconsolidated subsidiaries (2)									701.0	471.5	350.1
Payable to subsidiary									11.6	0.2	2.4
Payments to Acquire Interest in Subsidiaries and Affiliates										700.0	0.0	$ 0.0
Transfer from Investments									1,238.9	80.0
Proceeds from Dividends											15.0
Other liabilities	21.2				23.9				21.2	23.9
Investments in consolidated subsidiaries (3)	1,914.8				4,877.6				1,914.8	4,877.6
Short-term investments, at amortized cost (which approximates fair value)	57.2				12.5				57.2	12.5
WOBI
Cash flows from financing activities:
Equity in earnings from consolidated and unconsolidated subsidiaries (2)										(10.7)	$ (2.4)
Investments in consolidated subsidiaries (3)					$ (20.6)					$ (20.6)
Lone Tree Holdings Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Common shareholders’ equity	2,810.4								2,810.4
Cash flows from financing activities:
Other liabilities	14.1								14.1
Investments in consolidated subsidiaries (3)	964.4								964.4
Short-term investments, at amortized cost (which approximates fair value)	13.0								13.0
Lone Tree Holdings Ltd. | White Mountains Insurance Group Ltd.
Cash flows from financing activities:
Payable to subsidiary									1,256.7
Proceeds from divestiture of business									1,037.6
Non-cash reconciling item [Member] | White Mountains Insurance Group Ltd.
Cash flows from investing activities:
Issuance of debt from subsidiaries									94.2
intercompany balances [Member] | Lone Tree Holdings Ltd.
Cash flows from financing activities:
Other liabilities	$ 1,863.1								$ 1,863.1