CONSOLIDATED BALANCE SHEET (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Assets
Fixed maturity investments, at fair value	$ 4,914.0	$ 5,196.2
Short-term investments, at amortized cost (which approximates fair value)	648.0	630.6
Common equity securities, at fair value	1,110.7	1,029.7
Convertible fixed maturity investments, at fair value	84.8	127.4
Other long-term investments	300.0	294.2
Total investments	7,057.5	7,278.1
Cash (restricted: $74.4 and $249.8)	411.0	462.4
Reinsurance recoverable on unpaid losses	416.0	429.1
Reinsurance recoverable on paid losses	18.8	17.9
Insurance and reinsurance premiums receivable	684.9	556.3
Funds held by ceding companies	101.1	127.4
Investments in unconsolidated affiliates	331.7	387.9
Deferred acquisition costs	188.2	195.3
Deferred tax asset	555.5	569.6
Ceded unearned insurance and reinsurance premiums	116.4	91.8
Accrued investment income	33.7	45.9
Accounts receivable on unsettled investment sales	38.2	3.9
Other assets	397.3	503.0
Assets held for sale	1,957.2	2,226.8
Total assets	12,307.5	12,895.4
Liabilities
Loss and loss adjustment expense reserves	3,108.1	3,168.9
Unearned insurance and reinsurance premiums	1,010.3	924.1
Variable annuity benefit guarantee	143.7	441.5
Debt	676.3	751.2
Deferred tax liability	329.6	341.3
Accrued incentive compensation	161.6	159.0
Ceded reinsurance payable	153.6	116.5
Funds held under reinsurance treaties	89.1	43.7
Accounts payable on unsettled investment purchases	28.1	11.4
Other liabilities	362.7	452.8
Liabilities held for sale	1,957.2	2,226.8
Total liabilities	8,020.3	8,637.2
White Mountains’ common shareholders’ equity
White Mountains’ common shares	6.2	6.3
Paid-in surplus	1,040.4	1,050.9
Retained earnings	2,684.4	2,542.7
Accumulated other comprehensive income, after tax:
Equity in net unrealized (losses) gains from investments in unconsolidated affiliates	(23.5)	57.7
Net unrealized foreign currency translation gains	93.5	85.7
Pension liability and other	(11.0)	(11.5)
Total White Mountains’ common shareholders’ equity	3,790.0	3,731.8
Non-controlling interests
Total non-controlling interests	497.2	526.4
Total equity	4,287.2	4,258.2
Total liabilities and equity	12,307.5	12,895.4
OneBeacon
Non-controlling interests
Total non-controlling interests	263.3	251.4
Sirius Group [Member]
Non-controlling interests
Total non-controlling interests	250.0	250.0
HG Global
Non-controlling interests
Total non-controlling interests	16.6	16.6
BAM
Non-controlling interests
Total non-controlling interests	(80.7)	(36.0)
Other Noncontrolling Interest [Member]
Non-controlling interests
Total non-controlling interests	$ 48.0	$ 44.4