White Mountains Insurance (WTM) 10-Q 3 Aug 15 2015 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2015	Aug. 03, 2015
Document and Entity Information [Abstract]
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP LTD
Entity Central Index Key	776,867
Document Type	10-Q
Document Period End Date	Jun. 30, 2015
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		5,960,542
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q2

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Assets
Fixed maturity investments, at fair value	$ 4,874.5	$ 4,784.3
Short-term investments, at amortized cost (which approximates fair value)	840.5	871.7
Common equity securities, at fair value	713.8	801.6
Convertible fixed maturity and preferred investments, at fair value	4.5	20.5
Other long-term investments	390.1	407
Total investments	6,823.4	6,885.1
Cash (restricted: $20.9 and $23.7)	354.2	373.2
Reinsurance recoverable on unpaid losses	427.5	483.9
Reinsurance recoverable on paid losses	31.7	23.6
Insurance and reinsurance premiums receivable	711.9	547.7
Funds held by ceding entities	137.4	129
Investments in unconsolidated affiliates	397.2	414.4
Deferred acquisition costs	198.1	177.1
Deferred tax asset	440.2	456.1
Ceded unearned insurance and reinsurance premiums	172.4	94
Accrued investment income	33.7	34.5
Accounts receivable on unsettled investment sales	23.3	56.5
Goodwill and intangible assets	355.6	366.4
Other assets	322.7	356.1
Assets held for sale	0	58.1
Total assets	10,429.3	10,455.7
Liabilities
Loss and loss adjustment expense reserves	3,031.8	3,159.8
Unearned insurance and reinsurance premiums	1,110.8	955.3
Debt	748.5	746.6
Deferred tax liability	264.8	282.8
Accrued incentive compensation	124.9	184.6
Ceded reinsurance payable	189.6	105.7
Funds held under insurance and reinsurance contracts	145.4	138.9
Accounts payable on unsettled investment purchases	57.7	2.6
Other liabilities	297.3	341
Total liabilities	5,970.8	5,917.3
White Mountains’s common shareholders’ equity
White Mountains’ common shares	6	6
Paid-in surplus	1,025.2	1,028.7
Retained earnings	3,069.2	3,010.5
Accumulated other comprehensive income (loss), after tax:
Equity in net unrealized gains from investment in Symetra common shares	2	34.9
Net unrealized foreign currency translation losses	(123.7)	(79.8)
Pension liability and other	(4.1)	(4.6)
Total White Mountains’s common shareholders’ equity	3,974.6	3,995.7
Non-controlling interests
Total non-controlling interests	483.9	542.7
Total equity	4,458.5	4,538.4
Total liabilities and equity	10,429.3	10,455.7
OneBeacon
Non-controlling interests
Total non-controlling interests	255	258.4
Sirius Group
Non-controlling interests
Total non-controlling interests	250	250
Mutuals and reciprocals
Non-controlling interests
Total non-controlling interests	(146.2)	(134.3)
Other Noncontrolling Interest
Non-controlling interests
Total non-controlling interests	$ 125.1	$ 168.6

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Cash	$ 20.9	$ 23.7
Common shares, par value per share (in dollars per share)	$ 1	$ 1
Common shares, authorized shares	50,000,000	50,000,000
Common shares, issued shares	5,960,542	5,986,214
Common shares, outstanding shares	5,960,542	5,986,214

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE (LOSS) INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Revenues:
Earned insurance and reinsurance premiums	$ 537.3	$ 503.4	$ 1,031.6	$ 997
Net investment income	23.8	29.6	47.1	54.3
Net realized and unrealized investment (losses) gains	(61.1)	113.9	42.3	177.7
Other revenue	87.5	6.4	123.3	3
Total revenues	587.5	653.3	1,244.3	1,232
Expenses:
Loss and loss adjustment expenses	288.3	282.7	553.7	512
Insurance and Reinsurance Acquisition Expenses	105.3	98.9	200.9	194
Other Underwriting Expense	80.7	81.9	162.2	163.3
General and administrative expenses	108.1	70.3	224.5	120.6
Interest expense	10.9	10	22	20.1
Total expenses	593.3	543.8	1,163.3	1,010
Pre-tax income from continuing operations	(5.8)	109.5	81	222
Income tax benefit (expense)	2.7	(24.5)	(22.3)	(55.4)
Net income from continuing operations	(3.1)	85	58.7	166.6
Net (loss) income from sale of discontinued operations	(0.1)	2.7	7.9	2.7
Net loss from discontinued operations, net of tax	(0.2)	(0.1)	(0.3)	(0.6)
Income before equity in earnings of unconsolidated affiliates	(3.4)	87.6	66.3	168.7
Equity in earnings of unconsolidated affiliates, net of tax	6.8	12.5	14.1	26.3
Net income	3.4	100.1	80.4	195
Net loss (income) attributable to non-controlling interests	0.9	(4.6)	8.2	(4)
Net income attributable to White Mountains’s common shareholders	4.3	95.5	88.6	191
Other comprehensive (loss) income, net of tax:
Change in equity in net unrealized (losses) gains from investments in Symetra common shares, net of tax	(61.8)	32.9	(32.9)	69.4
Change in foreign currency translation, pension liability and other	44.2	(27.9)	(43.4)	(35.7)
Comprehensive income	(13.3)	100.5	12.3	224.7
Comprehensive income attributable to non-controlling interests	0	0	0	0
Comprehensive income attributable to White Mountains’s common shareholders	$ (13.3)	$ 100.5	$ 12.3	$ 224.7
Basic income per share
Continuing operations	$ 0.76	$ 15.08	$ 13.53	$ 30.61
Discontinued operations	(0.04)	0.43	1.27	0.34
Total consolidated operations	0.72	15.51	14.80	30.95
Diluted income per share
Continuing operations	0.76	15.08	13.53	30.61
Discontinued operations	(0.04)	0.43	1.27	0.34
Total consolidated operations	0.72	15.51	14.80	30.95
Dividends declared per White Mountains’s common share	$ 1	$ 1	$ 1	$ 1

CONSOLIDATED STATEMENTS OF CHAN

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions	Total	Common shares and paid-in surplus	Retained earnings	AOCI, after tax	Total	Non-controlling interest	Total Equity	Foreign Currency Gain (Loss) [Member]Common shares and paid-in surplus	Foreign Currency Gain (Loss) [Member]Retained earnings	Foreign Currency Gain (Loss) [Member]AOCI, after tax	Foreign Currency Gain (Loss) [Member]Total	Foreign Currency Gain (Loss) [Member]Non-controlling interest	Foreign Currency Gain (Loss) [Member]Total Equity	Pension liability and other accumulated comprehensive items [Member]Common shares and paid-in surplus	Pension liability and other accumulated comprehensive items [Member]Retained earnings	Pension liability and other accumulated comprehensive items [Member]AOCI, after tax	Pension liability and other accumulated comprehensive items [Member]Total	Pension liability and other accumulated comprehensive items [Member]Non-controlling interest	Pension liability and other accumulated comprehensive items [Member]Total Equity
Balances at Dec. 31, 2013		$ 1,051.1	$ 2,801.8	$ 52.1	$ 3,905	$ 491.8	$ 4,396.8
Increase (Decrease) in Shareholders' Equity
Net income	$ 195	0	191	0	191	4	195
Net change in unrealized gains from investments in unconsolidated affiliates	69.4	0	0	69.4	69.4	0	69.4
Other Comprehensive Income (Loss), Net of Tax								$ 0	$ 0	$ (35.9)	$ (35.9)	$ 0	$ (35.9)	$ 0	$ 0	$ 0.2	$ 0.2	$ 0	$ 0.2
Total comprehensive income (loss)	224.7	0	191	33.7	224.7	4	228.7
Dividends declared on common shares		0	(6.2)	0	(6.2)	0	(6.2)
Dividends to non-controlling interests	0	0	0	0	0	(19.9)	(19.9)
Repurchases and retirements of common shares		(8.8)	(21.5)	0	(30.3)	0	(30.3)
Issuances of common shares		2.9	0	0	2.9	0	2.9
Noncontrolling Interest, Period Increase (Decrease)		0	0	0	0	26.6	26.6
Amortization of restricted share awards		8.7	0	0	8.7	0.4	9.1
Balances at Jun. 30, 2014		1,053.9	2,965.1	85.8	4,104.8	502.9	4,607.7
Balances at Dec. 31, 2014	4,538.4	1,034.7	3,010.5	(49.5)	3,995.7	542.7	4,538.4
Increase (Decrease) in Shareholders' Equity
Net income	80.4	0	88.6	0	88.6	(8.2)	80.4
Net change in unrealized gains from investments in unconsolidated affiliates	(32.9)	0	0	(32.9)	(32.9)	0	(32.9)
Other Comprehensive Income (Loss), Net of Tax								$ 0	$ 0	$ (43.8)	$ (43.8)	$ (0.1)	$ (43.9)	$ 0	$ 0	$ 0.4	$ 0.4	$ 0.1	$ 0.5
Total comprehensive income (loss)	12.3	0	88.6	(76.3)	12.3	(8.2)	4.1
Dividends declared on common shares		0	(6)	0	(6)	0	(6)
Dividends to non-controlling interests	1	0	0	0	0	(21.9)	(21.9)
Repurchases and retirements of common shares		(8.8)	(23.9)	0	(32.7)	0	(32.7)
Issuances of common shares		0.9	0	0	0.9	0	0.9
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		0	0	0	0	(31.5)	(31.5)
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Changes, Additional Interest Issued to Parent		(2.2)	0	0	(2.2)	(2.7)	(4.9)
Noncontrolling Interest, Period Increase (Decrease)		0	0	0	0	5.8	5.8
Amortization of restricted share awards		6.6	0	0	6.6	(0.3)	6.3
Balances at Jun. 30, 2015	$ 4,458.5	$ 1,031.2	$ 3,069.2	$ (125.8)	$ 3,974.6	$ 483.9	$ 4,458.5

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Cash flows from operations:
Net income	$ 80.4	$ 195
Charges (credits) to reconcile net income to net cash used for operations:
Net realized and unrealized investment gains	(42.3)	(177.7)
Deferred income tax expense	1.8	22.6
Net (loss) income from discontinued operations	(7.6)	(2.1)
Gain on sale of subsidiary - Hamer and Citation	(16.1)	(0.7)
Amortization and depreciation	45	32.5
Undistributed equity in earnings from unconsolidated affiliates, net of tax	(14.1)	(26.3)
Other operating items:
Net change in loss and loss adjustment expense reserves	(79.7)	(39.7)
Net change in reinsurance recoverable on paid and unpaid losses	36	2.2
Net change in unearned insurance and reinsurance premiums	170.4	191
Net change in variable annuity benefit guarantee liabilities	1.1	(21)
Net change in variable annuity benefit guarantee derivative instruments	20.6	(5.6)
Net change in deferred acquisition costs	(23.6)	(16.5)
Net change in funds held by ceding entities	(13.1)	17.3
Net change in ceded unearned premiums	(82.6)	(49.6)
Net change in funds held under reinsurance treaties	9.8	(3.2)
Net change in insurance and reinsurance premiums receivable	(177.8)	(195.6)
Net change in ceded reinsurance payable	91.9	80
Net change in restricted cash	2.8	28.3
Net change in other assets and liabilities, net	(45.9)	(42.6)
Net cash used for operations - continuing operations	(43)	(11.7)
Net cash provided from (used for) operations - discontinued operations	7.3	(30.7)
Net cash used for operations	(35.7)	(42.4)
Cash flows from investing activities:
Net change in short-term investments	(17.1)	50.3
Sales of fixed maturity and convertible fixed maturity investments	2,076.2	2,340.9
Maturities, calls and paydowns of fixed maturity and convertible fixed maturity investments	256.1	277.6
Sales of common equity securities	576.6	127.6
Distributions and redemptions of other long-term investments	56.1	26.9
Sales of consolidated and unconsolidated affiliates, net of cash sold	24	12.8
Net Cash Provided by (Used in) Discontinued Operations	7.3	(30.7)
Purchases of other long-term investments	(33.3)	(17.7)
Purchases of common equity securities	(541.2)	(124)
Purchases of fixed maturity and convertible fixed maturity investments	(2,469.7)	(2,531.6)
Purchases of consolidated and unconsolidated affiliates, net of cash acquired	(2.4)	(32.2)
Net change in unsettled investment purchases and sales	88.4	(57.8)
Net dispositions (acquisitions) of property and equipment	52	(3.9)
Net cash provided from investing activities - continuing operations	73	38.2
Net cash (used for) provided from investing activities - discontinued operations	(7.3)	30.7
Net cash provided from investing activities	65.7	68.9
Cash flows from financing activities:
Draw down of debt and revolving line of credit	19.5	0
Repayment of debt and revolving line of credit	(17.6)	(0.2)
Payments on capital lease obligation	(2.8)	(2.6)
Cash dividends paid to the Company’s common shareholders	(6)	(6.2)
Cash dividends paid to OneBeacon Ltd.’s non-controlling common shareholders	(9.9)	(9.9)
Payments of Dividends for Preference Shares to Minority Interest	(9.4)	(9.4)
Common shares repurchased	(17.1)	(25.7)
OneBeacon Ltd. common shares repurchased and retired	(1.6)	0
Proceeds from Partnership Contribution	0	2.5
Distribution to non-controlling interest shareholders	(1)	0
Proceeds from (Payments for) Other Financing Activities	0	(4.9)
Collateral provided by interest rate cap counterparties	(0.6)	(4.5)
Payments to Acquire Additional Interest in Subsidiaries	(9.1)	0
Capital contributions from BAM members	11.5	7.7
Net cash used for financing activities - continuing operations	(44.1)	(53.2)
Net cash (used for) provided from financing activities - discontinued operations	0	0
Net cash used for financing activities	(44.1)	(53.2)
Effect of exchange rate changes on cash	(2.1)	3.2
Net change in cash during the period	(16.2)	(23.5)
Cash balances at beginning of period	349.5	326.7
Cash balances at end of period	333.3	303.2
Supplemental cash flows information:
Interest paid	(21.8)	(19.1)
Net income tax (payment to) refund from national governments	$ (17.1)	$ 1.6

CONSOLIDATED STATEMENTS OF CAS7

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Mar. 31, 2014
Statement of Cash Flows [Abstract]
Restricted cash balances, beginning of period	$ 23.7	$ 22.2	$ 56.1
Restricted cash balances, end of period	$ 20.9	$ 23.7	$ 22.2

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation These interim consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“GAAP”) and include the accounts of White Mountains Insurance Group, Ltd. (the “Company” or the “Registrant”), its subsidiaries (collectively, with the Company, “White Mountains”) and other entities required to be consolidated under GAAP. The Company is an exempted Bermuda limited liability company whose principal businesses are conducted through its insurance and reinsurance subsidiaries and affiliates. The Company’s headquarters is located at 14 Wesley Street, Hamilton, Bermuda HM 11, its principal executive office is located at 80 South Main Street, Hanover, New Hampshire 03755-2053 and its registered office is located at Clarendon House, 2 Church Street, Hamilton, Bermuda HM 11. White Mountains’s reportable segments are OneBeacon, Sirius Group, HG Global/BAM and Other Operations. The OneBeacon segment consists of OneBeacon Insurance Group, Ltd. (“OneBeacon Ltd.”), an exempted Bermuda limited liability company that owns a family of property and casualty insurance companies (collectively, “OneBeacon”). OneBeacon is a specialty property and casualty insurance writer that offers a wide range of insurance products in the United States through independent agencies, regional and national brokers, wholesalers and managing general agencies. During the third quarter of 2013, OneBeacon formed Split Rock Insurance, Ltd. (“Split Rock”), a Bermuda-based reinsurance company. As of June 30, 2015 and December 31, 2014 , White Mountains owned 75.3% of OneBeacon Ltd.’s outstanding common shares for both periods. In December 2014, OneBeacon completed the sale of its runoff business (the “Runoff Transaction”). Accordingly, OneBeacon’s runoff business is presented as discontinued operations. (See Note 17 - “Discontinued Operations”). In the second quarter of 2015, OneBeacon completed the sale of its building in Canton, MA for $58.0 million . The building was presented as held for sale at December 31, 2014. The Sirius Group segment consists of Sirius International Insurance Group, Ltd., an exempted Bermuda limited liability company, and its subsidiaries (collectively, “Sirius Group”). Sirius Group provides insurance and reinsurance products for property, accident and health, aviation and space, trade credit, marine, agriculture and certain other exposures on a worldwide basis through its primary subsidiaries, Sirius International Insurance Corporation (“Sirius International”), Sirius America Insurance Company (“Sirius America”) and Lloyds Syndicate 1945 (“Syndicate 1945”). Sirius Group also specializes in the acquisition and management of runoff insurance and reinsurance companies both in the United States and internationally through its White Mountains Solutions division (“WM Solutions”). On July 27, 2015, White Mountains announced it signed a definitive agreement to sell Sirius Group to CM International Holding PTE Ltd., the Singapore-based investment arm of China Minsheng Investment Corp., Ltd. (“CMI”). The purchase price will be paid in cash in an amount equal to 127.3% of Sirius’s closing date tangible common shareholder’s equity plus $10.0 million . Based on Sirius's tangible common shareholder's equity at December 31, 2014, the purchase price would be $2.2 billion . (See Note 19 - “Subsequent Events”) . The HG Global/BAM segment consists of HG Global Ltd. (“HG Global”) and the consolidated results of Build America Mutual Assurance Company (“BAM”). BAM is a municipal bond insurer domiciled in New York that was established in 2012 to provide insurance on bonds issued to support essential U.S. public purposes such as schools, utilities, core governmental functions and existing transportation facilities. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of surplus notes issued by BAM (the “BAM Surplus Notes”). HG Global, through its wholly-owned subsidiary, HG Re Ltd. (“HG Re”), also provides 15% -of-par, first loss reinsurance protection for policies underwritten by BAM. As of both June 30, 2015 and December 31, 2014 , White Mountains owned 96.9% of HG Global's preferred equity and 88.4% of its common equity. White Mountains does not have an ownership interest in BAM, which is a mutual insurance company owned by its members. However, GAAP requires White Mountains to consolidate BAM's results in its financial statements. BAM's results are attributed to non-controlling interests. White Mountains’s Other Operations segment consists of the Company and its intermediate holding companies, its wholly-owned investment management subsidiary, White Mountains Advisors LLC (“WM Advisors”), White Mountains’s variable annuity reinsurance business, White Mountains Life Reinsurance (Bermuda) Ltd. (“Life Re Bermuda”), which is in runoff with all of its contracts maturing by June 30, 2016, and its U.S.-based service provider, White Mountains Financial Services LLC (collectively, “WM Life Re”), and White Mountains’s ownership positions in Tranzact Holdings, LLC, Wobi Insurance Agency Ltd. (“Wobi”) and QL Holdings, LLC (together with its subsidiaries “MediaAlpha”). The Other Operations segment also includes Star & Shield Services LLC, Star & Shield Risk Management LLC (“SSRM”), and Star & Shield Claims Services LLC (collectively “Star & Shield”). Star & Shield provides management services for a fee to Star & Shield Insurance Exchange (“SSIE”), a reciprocal that is owned by its members, who are policyholders. As of June 30, 2015 , White Mountains holds $20.0 million of surplus notes issued by SSIE (the “SSIE Surplus Notes”) but does not have an ownership interest in SSIE. However, as a result of SSRM’s role as the attorney-in-fact to SSIE and the investment in SSIE’s surplus notes, White Mountains is required to consolidate SSIE in its GAAP financial statements. SSIE’s results do not affect White Mountains’s common shareholders’ equity as they are attributable to non-controlling interests. All significant intercompany transactions have been eliminated in consolidation. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. These interim financial statements include all adjustments considered necessary by management to fairly present the financial position, results of operations and cash flows of White Mountains. These interim financial statements may not be indicative of financial results for the full year and should be read in conjunction with the Company’s 2014 Annual Report on Form 10-K. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Refer to the Company’s 2014 Annual Report on Form 10-K for a complete discussion regarding White Mountains’s significant accounting policies. Recently Adopted Changes in Accounting Principles Qualified Affordable Housing Projects Effective January 1, 2015, White Mountains adopted ASU 2014-01, Accounting for Investments in Qualified Affordable Housing Projects (“QAHP”) (ASC 323). ASU 2014-01 allows investors in QAHP to make a policy election to use the proportional amortization method. Under the proportional amortization method, the investor amortizes the initial cost of the investment in proportion to the tax credits and other tax benefits received and recognizes the investment results, net of the related tax benefits, as a component of income tax expense. Prior to adoption, White Mountains accounted for its QAHP investment under the equity method and recognized its share of its QAHP investment's losses in investment income. White Mountains made the policy election to account for its investment in its QAHP investment using the proportional amortization method, applied retrospectively. Under the proportional amortization method, the cumulative loss recognized through December 31, 2014 and December 31, 2013 was $0.9 million and $0.5 million . The retrospective adoption resulted in an increase of $0.5 million and $0.9 million to net investment income and a net increase of $0.6 million and $1.2 million to income tax expense for the three and six months ended June 30, 2014. Footnote disclosures for prior year amounts have been amended to be consistent with the restated amounts described above. Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity On April 10, 2014, the FASB issued ASU 2014-08, Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity (ASC 205 and ASC 360) to reduce diversity in practice for reporting discontinued operations. ASU 2014-08 limits discontinued operations treatment to disposals that represent a strategic shift and that have a major effect on a reporting entity’s operations and financial results to be reported as discontinued operations. The revised guidance also requires expanded disclosure in the financial statements for discontinued operations as well as for disposals of significant components of an entity that do not qualify for discontinued operations presentation. White Mountains has not had any transactions that occurred since ASU 2014-08 became effective on December 15, 2014. Pushdown Accounting ASU 2014-17, Pushdown Accounting, a consensus of the FASB Emerging Issues Task Force (ASC 805) became effective upon its issuance on November 18, 2014 . The new guidance, which is applicable prospectively, gives an acquired non-public company the option to apply pushdown accounting in its separate company financial statements in the period in which it is acquired in a change of control transaction. Once pushdown accounting has been applied, the election is irreversible. Acquired entities that chose not to apply pushdown accounting at the time of acquisition, may apply pushdown accounting in a subsequent period as a change in accounting principle under ASC 250, Accounting Changes and Error Corrections . White Mountains has not had any acquisitions since AUS 2014-17 became effective. Unrecognized Tax Benefits Effective January 1, 2014, White Mountains adopted ASU 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists (ASC 740). The new ASU requires balance sheet presentation of an unrecognized tax benefit as a reduction of a deferred tax asset for a net operating loss (“NOL”) carryforward or tax credit carryforward rather than as a liability. The exception is in circumstances where a carryforward is not available to settle the additional taxes that might arise upon disallowance of the tax position under the tax law of the applicable jurisdiction. Prior to the issuance of ASU 2013-11, the guidance for unrecognized tax benefits under ASC 740 did not provide explicit guidance on whether an entity should present an unrecognized tax benefit as a liability or as a reduction of NOL carryforwards or other tax credits. In circumstances where an NOL carryforward is not available to offset settlement of any additional taxes arising from a disallowed tax position, the unrecognized tax benefit should be presented as a liability. The new guidance became effective for White Mountains on January 1, 2014. Adoption did not have any impact on White Mountains's financial condition, results of operations or cash flows or financial statement presentation. Recently Issued Accounting Pronouncements Short-Duration Contracts On May 21, 2015, the FASB issued ASU 2015-09, Disclosures about Short Duration Contracts (ASC 944) which requires expanded footnote disclosures about loss and loss adjustment expense (“LAE”) reserves. Under the new guidance, some disclosures currently presented outside of White Mountains’s financial statements, such as loss development tables and a reconciliation of loss development data to the loss and LAE reserves reflected on the balance sheet, will become part of the financial statement footnotes. In addition, the loss development tables required to be presented under the new ASU must be presented on a disaggregated basis by accident year rather than by reporting year as currently presented. Some of the expanded disclosures are new requirements, such as the disclosure of reserves for losses incurred but not reported (“IBNR”) plus expected development on reported claims, which must be presented by accident year on a disaggregated basis. The new guidance also requires new disclosures about claim frequency data together with descriptions of the approach used to measure that data. Qualitative descriptions of methodologies and assumptions used to develop IBNR estimates must be presented together with the disaggregated amounts of IBNR to which they relate, along with a discussion of any significant changes in methodology and assumptions and the related effect upon the loss reserves. The new guidance will be effective for annual periods beginning after December 15, 2015 and interim periods within annual periods beginning after December 15, 2016 with retrospective restatement of prior periods required. White Mountains will modify its financial statement footnote disclosures to conform to the requirements of ASU 2015-09 upon adoption, including revisions to prior year’s disclosures. Fair Value Measurements On May 1, 2015, the FASB issued ASU 2015-07, Fair Value Measurement - Disclosures for Investments in Certain Entities that Calculate Net Asset Value per Share (or Its Equivalent) (ASC 820) which eliminates the requirement to disclose the fair value hierarchy level for investments for which fair value is measured at net asset value using the practical expedient in ASC 820. White Mountains measures the fair value of its investments in hedge funds and private equity funds using this practical expedient and has classified those measurements within Level 3 of the fair value hierarchy. The new guidance is effective for fiscal years beginning after December 15, 2015 and interim periods within those fiscal years. Debt Issuance Costs On April 7, 2015, the FASB issued ASU 2015-03, Imputation of Interest (ASC 835) which requires debt issuance costs related to a recognized debt liability to be presented as a deduction from the carrying amount of the related debt, consistent with the treatment required for debt discounts. ASU 2015-03 is effective for annual and interim reporting periods beginning after December 15, 2015. White Mountains does not expect ASU 2015-03 to impact its financial position, results of operations, cash flows, presentation and disclosures. Amendments to Consolidation Analysis On February 18, 2015, the FASB issued ASU 2015-02, Amendments to the Consolidation Analysis (ASC 810) which amends the guidance for determining whether an entity is a variable interest entity (“VIE”). ASU 2015-02 eliminates the separate consolidation guidance for limited partnerships and with it, the presumption that a general partner should consolidate a limited partnership. In addition, ASU 2015-02 changes the guidance for determining if fee arrangements qualify as variable interests and the effect fee arrangements have on the determination of the primary beneficiary. ASU 2015-02 is effective for annual and interim reporting periods beginning after December 15, 2015 and must be applied retrospectively. White Mountains does not expect ASU 2015-02 to affect the consolidation analysis for any of its existing investments. Share-Based Compensation Awards On June 19, 2014, the FASB issued ASU 2014-12, Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period (ASC 718) . The new guidance is intended to eliminate diversity in practice for employee share-based awards containing performance targets that could be achieved after the requisite service period. Some reporting entities account for performance targets that can be achieved after the requisite service period as performance conditions that affect the vesting of the award while other reporting entities treat those performance targets as non-vesting conditions that affect the grant-date fair value of the award. The updated guidance requires that a performance target that affects vesting and that can be achieved after the requisite service period be treated as a performance condition. Compensation cost should be recognized in the period it becomes probable that the performance target will be achieved and should represent the compensation cost attributable to the periods for which service has been rendered. The new guidance is effective for annual and interim reporting periods beginning after December 15, 2015. White Mountains does not expect adoption to have a significant effect on its financial position, results of operations, cash flows, presentation or disclosures. Revenue Recognition On May 28, 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (ASC 606) , which modifies the guidance for revenue recognition. The scope of the new ASU excludes insurance contracts but is applicable to certain fee arrangements, such as third-party investment management fees charged by White Mountains Advisors, which were $2.0 million and $4.0 million for the three and six months ended June 30, 2015 and $3.0 million and $6.0 million for the three and six months ended June 30, 2014. White Mountains is in the process of evaluating the new guidance and has not yet determined the potential effect of adoption on its financial position, results of operations, or cash flows. ASU 2014-09 is effective for annual and interim reporting periods beginning after December 15, 2016.

Significant Transactions

Significant Transactions	6 Months Ended
	Jun. 30, 2015
Significant Transactions [Abstract]
Significant Transactions	Significant Transactions PassportCard On April 2, 2015, White Mountains closed on its 50 / 50 joint venture with DavidShield Life Insurance Agency (2000) Ltd. (“DavidShield”) for the development, marketing and distribution of PassportCard travel insurance (“PassportCard”), with White Mountains and DavidShield each contributing $21.0 million of assets to the newly formed entity, PPCI Global Ltd. OneTitle On April 22, 2015, White Mountains agreed to provide up to $13.0 million to OneTitle National Guaranty Company, Inc. (“OneTitle”) and will take a minority stake in the company. OneTitle is a licensed New York title insurance underwriter that works directly with attorneys, lenders, developers and homeowners. The transaction will close subject to regulatory approval. Tranzact On October 10, 2014, White Mountains acquired majority ownership of Tranzact, a leading provider of end-to-end, performance-driven customer acquisition solutions to the insurance sector. White Mountains acquired 63.2% of Tranzact for a purchase price of $177.7 million , representing an enterprise value of $281.2 million . Immediately following the closing, Tranzact completed a recapitalization that allowed for the return of $44.2 million in capital to White Mountains. As of the acquisition date, White Mountains recognized total assets acquired related to Tranzact of $332.8 million , including $41.4 million of tangible assets, $145.1 million of goodwill, and $146.3 million of other intangible assets; and total liabilities assumed of $108.7 million at their estimated fair values. MediaAlpha On March 14, 2014, White Mountains acquired 60% of the outstanding Class A common units of MediaAlpha. MediaAlpha is a California-based advertising technology company offering a transparent online exchange and sophisticated analytical tools that facilitate transactions between buyers (advertisers) and sellers (publishers) of insurance media (clicks and calls), including its own media inventory generated by owned and operated websites. Its exchange operates in four verticals: auto, home, health and life. White Mountains paid an initial purchase price of $28.1 million. The purchase price is subject to adjustment equal to 62.5% of the 2015 gross profit in excess of the 2013 gross profit. As of June 30, 2015 and December 31, 2014, White Mountains has recognized a $7.9 million liability for the contingent purchase price adjustment. After adjustment for the estimated contingent purchase price adjustment, White Mountains recognized total assets acquired related to MediaAlpha of $70.1 million , including $18.3 million of goodwill and $38.5 million of other intangible assets, and total liabilities assumed of $10.0 million , reflecting acquisition date fair values. Wobi On February 19, 2014, White Mountains acquired 54% of the outstanding common shares of Wobi for NIS 14.4 million (approximately $4.1 million based upon the foreign exchange spot rate at the date of acquisition). During 2014, in addition to the common shares, White Mountains also purchased NIS 31.5 million (approximately $8.5 million ) of newly-issued convertible preferred shares of Wobi. Wobi is the only price comparison/aggregation business in Israel, with an insurance carrier panel that represents 85% of the premiums written in the Israeli insurance market. Wobi sells four insurance lines of business, primarily personal auto, and operates as an agency, charging upfront commissions on all insurance policy sales. Wobi also offers a pension products comparison service for Israeli customers and is paid transaction fees when customers use the service to connect to companies that sell those pension products. As of the acquisition date, White Mountains recognized total assets acquired related to Wobi of $13.4 million , including $5.5 million of goodwill and $2.9 million of other intangible assets; and total liabilities assumed of $0.7 million at their estimated fair values. During the second quarter of 2015, White Mountains increased its ownership interest in Wobi through the purchase of shares from a non-controlling interest shareholder for NIS 35.0 million (approximately $ 9.1 million ) and newly-issued convertible preferred shares for NIS 25.0 million (approximately $ 6.6 million ). As of June 30, 2015, White Mountains’s ownership share was 95.3% on a fully converted basis. On February 23, 2015, Wobi acquired 56.2% of the outstanding share capital of Tnuva Finansit Ltd. (“Cashboard”) for NIS 9.5 million (approximately $2.4 million ). The acquisition of Cashboard accelerated Wobi's development of its pension products comparison service. As of the acquisition date, Wobi recognized total assets acquired of $5.5 million , including $0.3 million of goodwill and $2.8 million of other intangible assets; and total liabilities assumed of $1.2 million at their estimated fair values. During the second quarter of 2015, Wobi purchased newly issued common shares of Cashboard for NIS 10.0 million (approximately $2.6 million ), which increased its ownership interest in Cashboard to 68.3% . Star & Shield On January 31, 2014, White Mountains acquired certain assets and liabilities of Star & Shield Holdings LLC, including SSRM, the attorney-in-fact for SSIE, for a purchase price of $1.8 million . White Mountains owns $20.0 million of surplus notes issued by SSIE. Principal and interest on the surplus notes are payable to White Mountains only with approval from the Florida Office of Insurance Regulation. SSIE is a Florida-domiciled reciprocal insurance exchange providing private passenger auto insurance to the public safety community and their families. SSIE is a variable interest entity (“VIE”). As a result of SSRM’s role as the attorney-in-fact to SSIE and the investment in SSIE’s surplus notes, White Mountains is required to consolidate SSIE. At June 30, 2015 and December 31, 2014, consolidated amounts included total assets of $13.4 million and $13.5 million and total liabilities of $27.8 million and $25.9 million , respectively of SSIE. The surplus notes purchased by White Mountains and the corresponding liability included in SSIE's liabilities are eliminated in consolidation. For the three months ended June 30, 2015 and 2014, SSIE had pre-tax losses of $0.8 million and $5.2 million that were recorded in net loss attributable to non-controlling interests. For the six months ended June 30, 2015 and 2014, SSIE had pre-tax losses of $1.9 million and $9.7 million that were recorded in net loss attributable to non-controlling interests.

Loss and Loss Adjustment Expens

Loss and Loss Adjustment Expense Reserves	6 Months Ended
	Jun. 30, 2015
Insurance Loss Reserves [Abstract]
Loss and Loss Adjustment Expense Reserves	Loss and Loss Adjustment Expense Reserves The following table summarizes the loss and loss adjustment expense (“LAE”) reserve activities of White Mountains’s insurance and reinsurance subsidiaries for the three and six months ended June 30, 2015 and 2014 : Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Gross beginning balance $ 3,035.0 $ 3,048.4 $ 3,159.8 $ 3,079.3 Less beginning reinsurance recoverable on unpaid losses (436.9 ) (425.2 ) (483.9 ) (428.1 ) Net loss and LAE reserves 2,598.1 2,623.2 2,675.9 2,651.2 Loss and LAE reserves consolidated — SSIE — — — 13.6 Loss and LAE incurred relating to: Current year losses 300.0 278.4 566.8 518.9 Prior year losses (11.7 ) 4.3 (13.1 ) (6.9 ) Total incurred losses and LAE 288.3 282.7 553.7 512.0 Foreign currency translation adjustment to loss and LAE reserves 7.5 (.5 ) (14.9 ) 1.3 Loss and LAE paid relating to: Current year losses (72.9 ) (65.8 ) (113.5 ) (100.8 ) Prior year losses (216.7 ) (219.5 ) (496.9 ) (457.2 ) Total loss and LAE payments (289.6 ) (285.3 ) (610.4 ) (558.0 ) Net ending balance 2,604.3 2,620.1 2,604.3 2,620.1 Plus ending reinsurance recoverable on unpaid losses 427.5 433.2 427.5 433.2 Gross ending balance $ 3,031.8 $ 3,053.3 $ 3,031.8 $ 3,053.3 Loss and LAE incurred relating to prior year losses for the three and six months ended June 30, 2015 For the three and six months ended June 30, 2015 , White Mountains experienced net favorable loss reserve development of $11.7 million and $13.1 million . For the three and six months ended June 30, 2015 , Sirius Group had net favorable loss reserve development of $11.4 million , and $10.9 million primarily due to reductions in reserves for prior year catastrophes and run-off casualty business. During the three months ended June 30, 2015, OneBeacon experienced no net loss and LAE reserve development. For the six months ended June 30, 2015, OneBeacon had net favorable loss reserve development of $1.8 million . For the three and six months ended June 30, 2015 , SSIE had net favorable loss reserve development of $0.3 million and $0.4 million . Loss and LAE incurred relating to prior year losses for the three and six months ended June 30, 2014 For the three and six months ended June 30, 2014 , White Mountains experienced net unfavorable loss reserve development of $4.3 million and net favorable loss reserve development of $6.9 million . For the three and six months ended June 30, 2014 , OneBeacon had net unfavorable loss reserve development of $8.4 million and $7.0 million primarily related to a few large losses in OneBeacon Other Professional Lines and Management Liability, as well as OneBeacon Entertainment, OneBeacon Government Risks and OneBeacon Accident Group, partially offset by favorable loss reserve development in OneBeacon Specialty Property. For the three and six months ended June 30, 2014 , Sirius Group had net favorable loss reserve development of $6.3 million and $16.1 million primarily due to decreases in property loss reserves, including reductions for prior period catastrophe losses. For both the three and six months ended June 30, 2014 , SSIE had net unfavorable loss reserve development of $2.2 million .

Third Party Reinsurance

Third Party Reinsurance	6 Months Ended
	Jun. 30, 2015
Reinsurance Disclosures [Abstract]
Third Party Reinsurance	Third Party Reinsurance In the normal course of business, White Mountains’s insurance and reinsurance subsidiaries may seek to limit losses that may arise from catastrophes or other events by reinsuring with third party reinsurers. White Mountains remains liable for risks reinsured in the event that the reinsurer does not honor its obligations under reinsurance contracts. OneBeacon At June 30, 2015 , OneBeacon had $20.9 million and $147.1 million of reinsurance recoverables on paid and unpaid losses. At December 31, 2014 , OneBeacon had $12.2 million and $161.6 million of reinsurance recoverables on paid and unpaid losses. The reinsurance balances associated with the Runoff Business are included in discontinued operations. (See Note 17 - “Discontinued Operations” ). Reinsurance contracts do not relieve OneBeacon of its obligation to its policyholders. OneBeacon is selective with its reinsurers, placing reinsurance with only those reinsurers having a strong financial condition. OneBeacon monitors the financial strength and ratings of its reinsurers on an ongoing basis. Uncollectible amounts related to the ongoing specialty business historically have not been significant. Except as discussed below, there have been no material changes to OneBeacon's reinsurance coverage as discussed in Note 4 —“Reinsurance” in White Mountains’s 2014 Annual Report on Form 10-K. Effective January 1, 2015, OneBeacon purchased an aggregate stop loss on its multiple peril crop insurance (“MPCI”) portfolio, providing 52.0% coverage in excess of a 98.0% loss ratio on premiums covered by the contract and a separate aggregate stop loss providing 80% coverage in excess of a 100% loss ratio on its crop-hail portfolio. OneBeacon also purchased a new quota share contract on 30% of its MPCI portfolio. Effective May 1, 2015, OneBeacon renewed its property catastrophe reinsurance program through April 30, 2016. The program provides coverage for OneBeacon's property business as well as certain acts of terrorism. Under the program, the first $20.0 million of losses resulting from any single catastrophe are retained, and 95.0% of the next $10.0 million of losses and 100.0% of the next $100.0 million of losses resulting from the catastrophe are reinsured. The part of the catastrophe loss in excess of $ 130.0 million would be retained in full. In the event of a catastrophe, OneBeacon's property catastrophe reinsurance program is reinstated for the remainder of the original contract term by paying a reinstatement premium that is based on the percentage of coverage reinstated and the original property catastrophe coverage premium. Also effective May 1, 2015, OneBeacon lowered its retention on its property-per-risk reinsurance program from $5.0 million to $3.0 million . Effective June 1, 2015, OneBeacon lowered its retentions on certain casualty and healthcare treaties from $5.0 million to $3.0 million . Sirius Group At June 30, 2015 , Sirius Group had $10.8 million and $280.3 million of reinsurance recoverables on paid and unpaid losses that will become recoverable if claims are paid in accordance with current reserve estimates. At December 31, 2014 , Sirius Group had $11.4 million and $322.2 million of reinsurance recoverables on paid and unpaid losses. Because retrocessional reinsurance contracts do not relieve Sirius Group of its obligation to its insureds, the collectability of balances due from Sirius Group’s reinsurers is important to its financial strength. Sirius Group monitors the financial strength and ratings of retrocessionaires on an ongoing basis. Uncollectible amounts historically have not been significant. In connection with the sale of Sirius Group, White Mountains caused Sirius Group to purchase several industry loss warranty (“ILW”) contracts to augment its overall retrocessional program in order to reduce the risk of loss from major catastrophes subsequent to the signing of the definitive agreement to sell Sirius Group. (See Note 19 - “Subsequent Events” ).

Investment Securities

Investment Securities	6 Months Ended
	Jun. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Investment Securities	Investment Securities White Mountains’s invested assets consist of securities and other long-term investments held for general investment purposes. The portfolio of investment securities includes fixed maturity investments, short-term investments, common equity securities, and convertible fixed maturity and preferred investments which are all classified as trading securities. Trading securities are reported at fair value as of the balance sheet date. Realized and unrealized investment gains and losses on trading securities are reported in pre-tax revenues. Income on mortgage-backed and asset-backed securities is recognized using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When actual prepayments differ significantly from anticipated prepayments, the estimated economic life is recalculated and the remaining unamortized premium or discount is amortized prospectively over the remaining economic life. Realized investment gains and losses resulting from sales of investment securities are accounted for using the specific identification method. Premiums and discounts on all fixed maturity investments are amortized or accreted to income over the anticipated life of the investment. Short-term investments consist of money market funds, certificates of deposit and other securities which, at the time of purchase, mature or become available for use within one year. Short-term investments are carried at amortized or accreted cost, which approximated fair value as of June 30, 2015 and December 31, 2014 . Other long-term investments consist primarily of hedge funds, private equity funds, surplus note investments, private equity securities and limited liability companies and other investments in limited partnerships. Net Investment Income Pre-tax net investment income for the three and six months ended June 30, 2015 and 2014 consisted of the following: Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Investment income: Fixed maturity investments $ 23.4 $ 25.6 $ 46.5 $ 48.0 Short-term investments .7 .6 1.3 1.2 Common equity securities 3.3 5.5 6.7 10.9 Convertible fixed maturity and preferred investments .1 .8 .3 1.2 Other long-term investments .9 1.9 .8 2.6 Interest on funds held under reinsurance treaties — — (.1 ) — Total investment income 28.4 34.4 55.5 63.9 Third-party investment expenses (4.6 ) (4.8 ) (8.4 ) (9.6 ) Net investment income, pre-tax $ 23.8 $ 29.6 $ 47.1 $ 54.3 Net Realized and Unrealized Investment Gains and Losses Net realized and unrealized investment gains and losses for the three and six months ended June 30, 2015 and 2014 consisted of the following: Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Net realized investment gains, pre-tax $ 112.9 $ 30.5 $ 165.7 $ 52.2 Net unrealized investment (losses) gains, pre-tax (174.0 ) 83.4 (123.4 ) 125.5 Net realized and unrealized investment (losses) gains, pre-tax (61.1 ) 113.9 42.3 177.7 Income tax benefit (expense) attributable to net realized and unrealized investment gains (losses) 16.5 (23.0 ) (9.8 ) (38.7 ) Net realized and unrealized investment (losses) gains, after tax $ (44.6 ) $ 90.9 $ 32.5 $ 139.0 Net realized investment gains (losses) Net realized investment gains (losses) for the three and six months ended June 30, 2015 and 2014 consisted of the following: Three Months Ended Three Months Ended June 30, 2015 June 30, 2014 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ 2.7 $ 14.3 $ 17.0 $ 5.7 $ 3.0 $ 8.7 Short-term investments — 5.7 5.7 — — — Common equity securities 65.9 7.2 73.1 19.4 — 19.4 Convertible fixed maturity and preferred investments .6 — .6 1.5 — 1.5 Other long-term investments 16.9 — 16.9 .6 .3 .9 Forward contracts (.4 ) — (.4 ) — — — Net realized investment gains, pre-tax 85.7 27.2 112.9 27.2 3.3 30.5 Income tax (expense) attributable to net realized investment gains (24.6 ) (6.4 ) (31.0 ) (5.3 ) (1.0 ) (6.3 ) Net realized investment gains, after tax $ 61.1 $ 20.8 $ 81.9 $ 21.9 $ 2.3 $ 24.2 Six Months Ended Six Months Ended June 30, 2015 June 30, 2014 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ 12.4 $ 35.5 $ 47.9 $ 10.1 $ (.5 ) $ 9.6 Short-term investments — 9.3 9.3 — — — Common equity securities 90.3 8.3 98.6 38.3 (.1 ) 38.2 Convertible fixed maturity and preferred investments (3.6 ) — (3.6 ) 3.9 — 3.9 Other long-term investments 12.8 1.1 13.9 .3 .3 .6 Forward contracts (.4 ) — (.4 ) (.1 ) — (.1 ) Net realized investment gains (losses), pre-tax 111.5 54.2 165.7 52.5 (.3 ) 52.2 Income tax (expense) attributable to net realized investment gains (losses) (31.1 ) (13.1 ) (44.2 ) (9.9 ) (.1 ) (10.0 ) Net realized investment gains (losses), after tax $ 80.4 $ 41.1 $ 121.5 $ 42.6 $ (.4 ) $ 42.2 Net unrealized investment gains (losses) The following table summarizes net unrealized investment gains (losses) for the three and six months ended June 30, 2015 and 2014 : Three Months Ended Three Months Ended June 30, 2015 June 30, 2014 Millions Net unrealized (losses) Net Total net unrealized (losses) reflected in earnings Net unrealized gains (losses) Net Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ (34.7 ) $ (39.0 ) $ (73.7 ) $ 24.9 $ 28.2 $ 53.1 Short-term investments — — — — — — Common equity securities (63.9 ) (8.2 ) (72.1 ) 21.0 1.4 22.4 Convertible fixed maturity and preferred investments — (.1 ) (.1 ) (2.4 ) .3 (2.1 ) Other long-term investments (29.1 ) 1.0 (28.1 ) 8.7 1.3 10.0 Forward contracts — — — — — — Net unrealized investment (losses) (127.7 ) (46.3 ) (174.0 ) 52.2 31.2 83.4 Income tax benefit (expense) 36.0 11.5 47.5 (9.8 ) (6.9 ) (16.7 ) Net unrealized investment $ (91.7 ) $ (34.8 ) $ (126.5 ) $ 42.4 $ 24.3 $ 66.7 Six Months Ended Six Months Ended June 30, 2015 June 30, 2014 Millions Net unrealized (losses) gains Net Total net unrealized (losses) gains reflected in earnings Net unrealized gains (losses) Net Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ (22.6 ) $ 1.2 $ (21.4 ) $ 44.3 $ 40.4 $ 84.7 Short-term investments — — — — — — Common equity securities (68.1 ) (7.5 ) (75.6 ) 25.0 1.9 26.9 Convertible fixed maturity and preferred investments .7 — .7 (1.7 ) .3 (1.4 ) Other long-term investments (28.2 ) 1.1 (27.1 ) 13.6 1.7 15.3 Forward contracts — — — — — — Net unrealized investment (losses) gains, pre-tax (118.2 ) (5.2 ) (123.4 ) 81.2 44.3 125.5 Income tax benefit (expense) attributable to net unrealized investment (losses) gains 33.2 1.2 34.4 (18.9 ) (9.8 ) (28.7 ) Net unrealized investment (losses) gains, after tax $ (85.0 ) $ (4.0 ) $ (89.0 ) $ 62.3 $ 34.5 $ 96.8 The following table summarizes the amount of total pre-tax unrealized investment (losses) gains included in earnings for Level 3 investments for the three and six months ended June 30, 2015 and 2014 : Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Fixed maturity investments $ .3 $ .2 $ (.4 ) $ .4 Common equity securities 4.9 2.0 3.1 2.8 Convertible fixed maturity and preferred investments .7 3.2 .8 3.2 Other long-term investments (27.2 ) 8.9 (30.0 ) 15.0 Total unrealized investment (losses) gains, pre-tax - Level 3 investments $ (21.3 ) $ 14.3 $ (26.5 ) $ 21.4 Investment Holdings The cost or amortized cost, gross unrealized investment gains and losses, net foreign currency gains and losses, and carrying values of White Mountains’s fixed maturity investments as of June 30, 2015 and December 31, 2014 were as follows: June 30, 2015 Millions Cost or amortized cost (1) Gross unrealized gains Gross unrealized losses Net foreign currency (losses) gain (1) Carrying value U.S. Government and agency obligations $ 183.9 $ .3 $ (.2 ) $ (.4 ) $ 183.6 Debt securities issued by corporations 2,232.7 27.7 (7.0 ) 57.7 2,311.1 Municipal obligations 118.0 .8 (1.1 ) — 117.7 Mortgage-backed and asset-backed securities 2,057.4 9.2 (4.7 ) 15.5 2,077.4 Foreign government, agency and provincial obligations 98.9 .3 (1.1 ) .6 98.7 Preferred stocks 79.5 6.3 — .2 86.0 Total fixed maturity investments $ 4,770.4 $ 44.6 $ (14.1 ) $ 73.6 $ 4,874.5 (1) The cost of investment securities is determined using the functional currency of the White Mountains consolidated entity that holds the investment. As a result, the table includes unrealized foreign currency gains from U.S. dollar denominated investments held at Sirius Group. December 31, 2014 Millions Cost or amortized cost (1) Gross unrealized gains Gross unrealized losses Net foreign currency gains (losses) (1) Carrying value U.S. Government and agency obligations $ 184.7 $ .1 $ (.3 ) $ 3.6 $ 188.1 Debt securities issued by corporations 2,221.3 45.2 (5.1 ) 49.8 2,311.2 Municipal obligations 82.0 1.4 (.2 ) — 83.2 Mortgage-backed and asset-backed securities 1,811.1 7.6 (3.5 ) 25.7 1,840.9 Foreign government, agency and provincial obligations 274.6 4.2 (1.0 ) (2.7 ) 275.1 Preferred stocks 79.6 6.1 — .1 85.8 Total fixed maturity investments $ 4,653.3 $ 64.6 $ (10.1 ) $ 76.5 $ 4,784.3 (1) The cost of investment securities is determined using the functional currency of the White Mountains consolidated entity that holds the investment. As a result, the table includes unrealized foreign currency gains from U.S. dollar denominated investments held at Sirius Group. The cost or amortized cost, gross unrealized investment gains and losses, net foreign currency gains and losses, and carrying values of White Mountains’s common equity securities, convertible fixed maturity and preferred investments and other long-term investments as of June 30, 2015 and December 31, 2014 were as follows: June 30, 2015 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign Carrying value Common equity securities $ 656.7 $ 69.1 $ (12.2 ) $ .2 $ 713.8 Convertible fixed maturity and preferred investments $ 3.1 $ 1.4 $ — $ — $ 4.5 Other long-term investments $ 346.0 $ 54.0 $ (11.9 ) $ 2.0 $ 390.1 December 31, 2014 Millions Cost or Gross Gross Net foreign Carrying value Common equity securities $ 633.6 $ 175.1 $ (5.2 ) $ (1.9 ) $ 801.6 Convertible fixed maturity and preferred investments $ 19.1 $ .9 $ (.2 ) $ .7 $ 20.5 Other long-term investments $ 343.3 $ 73.0 $ (10.9 ) $ 1.6 $ 407.0 Other Long-term Investments Other long-term investments consist of the following as of June 30, 2015 and December 31, 2014 : Carrying Value at Millions June 30, 2015 December 31, 2014 Hedge funds and private equity funds, at fair value (1) $ 210.8 $ 242.9 Surplus notes investments, at fair value (1) 66.3 65.1 Private equity securities and limited liability companies, at fair value (1)(2) 89.9 69.7 Tax advantaged federal affordable housing development fund (3) 15.8 16.8 Partnership investments accounted for under the equity method 3.9 5.2 Other (1) 3.4 7.3 Total other-long term investments $ 390.1 $ 407.0 (1) See Fair Value Measurements by Level table. (2) On April 2, 2015, White Mountains invested $21.0 in PassportCard. White Mountains has chosen to take the fair value election for its investment. (3) Fund accounted for using the proportional amortization method. Hedge Funds and Private Equity Funds White Mountains holds investments in hedge funds and private equity funds, which are included in other long-term investments. The fair value of these investments has been estimated using the net asset value of the funds. As of June 30, 2015 , White Mountains held investments in 14 hedge funds and 29 private equity funds. The largest investment in a single fund was $25.0 million as of June 30, 2015 . The following table summarizes investments in hedge funds and private equity funds by investment objective and sector as of June 30, 2015 and December 31, 2014 : June 30, 2015 December 31, 2014 Millions Fair Value Unfunded Commitments Fair Value Unfunded Commitments Hedge funds Long/short equity REIT $ 20.5 — $ 20.3 — Long/short credit & distressed 19.2 — 21.4 — Long/short equity banks and financial 15.9 $ — 29.9 $ — Other 22.1 — 20.4 — Total hedge funds 77.7 — 92.0 — Private equity funds Energy infrastructure & services 45.7 9.9 59.6 11.0 Manufacturing/Industrial 25.9 2.9 23.2 7.3 Multi-sector 24.5 4.4 24.2 5.3 Aerospace/Defense/Government 18.7 32.2 20.7 5.1 Healthcare 6.3 .8 6.1 2.8 Private equity secondaries 7.4 3.2 8.5 3.1 Insurance 2.1 41.3 2.1 41.2 Real estate 1.8 .1 3.6 3.3 Venture capital .3 — 1.4 .3 International multi-sector, Europe .4 — 1.5 2.3 Total private equity funds 133.1 94.8 150.9 81.7 Total hedge funds and private equity funds included in other long-term investments $ 210.8 $ 94.8 $ 242.9 $ 81.7 Redemption of investments in certain hedge funds is subject to restrictions including lock-up periods where no redemptions or withdrawals are allowed, restrictions on redemption frequency and advance notice periods for redemptions. Amounts requested for redemptions remain subject to market fluctuations until the redemption effective date, which generally falls at the end of the defined redemption period. The following summarizes the June 30, 2015 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds: Notice Period Millions Redemption frequency 30-59 days notice 60-89 days notice 90-119 days notice 120+ days notice Total Monthly $ 5.2 $ — $ — $ — $ 5.2 Quarterly 17.8 19.2 — 8.0 45.0 Semi-annual — 22.5 — — 22.5 Annual — — 4.9 .1 5.0 Total $ 23.0 $ 41.7 $ 4.9 $ 8.1 $ 77.7 Certain of the hedge fund investments in which White Mountains is invested are no longer active and are in the process of disposing of their underlying investments. Distributions from such funds are remitted to investors as the fund’s underlying investments are liquidated. As of June 30, 2015 , distributions of $2.0 million were outstanding from these investments. The actual amount of the final distribution remittances remain subject to market fluctuations. The date at which such remittances will be received is not determinable as of June 30, 2015 . White Mountains has also submitted redemption requests for certain of its investments in active hedge funds. As of June 30, 2015 , redemptions of $10.2 million are outstanding and are subject to market fluctuations. The date at which such redemptions will be received is not determinable as of June 30, 2015 . Redemptions are recorded as receivables when the investment is no longer subject to market fluctuations. Investments in private equity funds are generally subject to a “lock-up” period during which investors may not request a redemption. Distributions prior to the expected termination date of the fund may be limited to dividends or proceeds arising from the liquidation of the fund’s underlying investments. In addition, certain private equity funds provide an option to extend the lock-up period at either the sole discretion of the fund manager or upon agreement between the fund and the investors. As of June 30, 2015 , investments in private equity funds were subject to lock-up periods as follows: Millions 1-3 years 3 – 5 years 5 – 10 years >10 years Total Private Equity Funds — expected lock-up period remaining $27.0 $29.5 $72.5 $4.1 $133.1 The Prospector Offshore Fund is a hedge fund managed by Prospector Partners LLC (“Prospector”) that pursues investment opportunities in a variety of equity and equity-related instruments, with a principle focus on the financial services sector and a special emphasis on the insurance industry. White Mountains had determined that the Prospector Offshore Fund was a VIE that White Mountains was required to consolidate. In the second quarter of 2015, White Mountains redeemed the Prospector-managed hedge funds (the Prospector Offshore Fund and the Prospector Partners Fund LP, together the “ Prospector Funds”). As of December 31, 2014, White Mountains’s investment in the Prospector Offshore Fund was $64.9 million . As of December 31, 2014, White Mountains consolidated total assets of $135.8 million and total liabilities of $39.8 million of the Prospector Offshore Fund. As of December 31, 2014, White Mountains also recorded non-controlling interest of $31.1 million in the Prospector Offshore Fund. Fair value measurements as of June 30, 2015 Fair value measurements are categorized into a hierarchy that distinguishes between inputs based on market data from independent sources (“observable inputs”) and a reporting entity's internal assumptions based upon the best information available when external market data is limited or unavailable (“unobservable inputs”). Quoted prices in active markets for identical assets or liabilities have the highest priority (“Level 1”), followed by observable inputs other than quoted prices, including prices for similar but not identical assets or liabilities (“Level 2”) and unobservable inputs, including the reporting entity's estimates of the assumptions that market participants would use, having the lowest priority (“Level 3”). Investments valued using Level 1 inputs include fixed maturity investments, primarily investments in U.S. Treasuries, common equity securities and short-term investments, which include U.S. Treasury Bills. Investments valued using Level 2 inputs are primarily comprised of fixed maturity investments, which have been disaggregated into classes, including debt securities issued by corporations, municipal obligations, mortgage and asset-backed securities, foreign government obligations and preferred stocks. Fair value estimates for investments that trade infrequently and have few or no observable market prices are classified as Level 3 measurements. Level 3 fair value estimates based upon unobservable inputs include White Mountains's investments in surplus notes, hedge funds and private equity funds, as well as certain investments in fixed maturity investments, common equity securities, and convertible fixed and preferred maturity investments where quoted market prices are unavailable or are not considered reasonable. White Mountains determines when transfers between levels have occurred as of the beginning of the period. White Mountains uses brokers and outside pricing services to assist in determining fair values. For investments in active markets, White Mountains uses the quoted market prices provided by outside pricing services to determine fair value. The outside pricing services White Mountains uses have indicated that they will only provide prices where observable inputs are available. In circumstances where quoted market prices are unavailable or are not considered reasonable, White Mountains estimates the fair value using industry standard pricing models and observable inputs such as benchmark yields, reported trades, broker-dealer quotes, issuer spreads, benchmark securities, bids, offers, prepayment speeds, reference data including research publications and other relevant inputs. Given that many fixed maturity investments do not trade on a daily basis, the outside pricing services evaluate a wide range of fixed maturity investments by regularly drawing parallels from recent trades and quotes of comparable securities with similar features. The characteristics used to identify comparable fixed maturity investments vary by asset type and take into account market convention. White Mountains's process to assess the reasonableness of the market prices obtained from the outside pricing sources covers substantially all of its fixed maturity investments and includes, but is not limited to, evaluation of model pricing methodologies, review of the pricing services' quality control processes and procedures on at least an annual basis, comparison of market prices to prices obtained from different independent pricing vendors on at least a semi-annual basis, monthly analytical reviews of certain prices and review of assumptions utilized by the pricing service for selected measurements on an ad hoc basis throughout the year. White Mountains also performs back-testing of selected purchases and sales activity to determine whether there are any significant differences between the market price used to value the security prior to purchase or sale and the actual purchase or sale price on an ad-hoc basis throughout the year. Prices provided by the pricing services that vary by more than 5% and $1.0 million from the expected price based on these procedures are considered outliers. Also considered outliers are prices that haven’t changed from period to period and prices that have trended unusually compared to market conditions. In circumstances where the results of White Mountains's review process does not appear to support the market price provided by the pricing services, White Mountains challenges the price. If White Mountains cannot gain satisfactory evidence to support the challenged price, it relies upon its own pricing methodologies to estimate the fair value of the security in question. The valuation process above is generally applicable to all of White Mountains’s fixed maturity investments. The techniques and inputs specific to asset classes within White Mountains’s fixed maturity investments for Level 2 securities that use observable inputs are as follow: Debt securities issued by corporations: The fair value of debt securities issued by corporations is determined from an evaluated pricing model that uses information from market sources and integrates relative credit information, observed market movements, and sector news. Key inputs include benchmark yields, reported trades, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including sector, coupon, credit quality ratings, duration, credit enhancements, early redemption features and market research publications. Municipal obligations: The fair value of municipal obligations is determined from an evaluated pricing model that uses information from market makers, brokers-dealers, buy-side firms, and analysts along with general market information. Key inputs include benchmark yields, reported trades, issuer financial statements, material event notices and new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including type, coupon, credit quality ratings, duration, credit enhancements, geographic location and market research publications. Mortgage and asset-backed securities: The fair value of mortgage and asset-backed securities is determined from an evaluated pricing model that uses information from market sources and leveraging similar securities. Key inputs include benchmark yields, reported trades, underlying tranche cash flow data, collateral performance, plus new issue data, as well as broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including issuer, vintage, loan type, collateral attributes, prepayment speeds, default rates, recovery rates, cash flow stress testing, credit quality ratings and market research publications. Foreign government obligations: The fair value of foreign government obligations is determined from an evaluated pricing model that uses feeds from data sources in each respective country, including active market makers and inter-dealer brokers. Key inputs include benchmark yields, reported trades, broker-dealer quotes, two-sided markets, benchmark securities, bids, offers, local exchange prices, foreign exchange rates and reference data including coupon, credit quality ratings, duration and market research publications. Preferred stocks: The fair value of preferred stocks is determined from an evaluated pricing model that calculates the appropriate spread over a comparable security for each issue. Key inputs include exchange prices (underlying and common stock of same issuer), benchmark yields, reported trades, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, and reference data including sector, coupon, credit quality ratings, duration, credit enhancements, early redemption features and market research publications. Level 3 valuations are generated from techniques that use assumptions not observable in the market. These unobservable assumptions reflect White Mountains’s assumptions that market participants would use in valuing the investment. Generally, certain securities may start out as Level 3 when they are originally issued but as observable inputs become available in the market, they may be reclassified to Level 2. White Mountains employs a number of procedures to assess the reasonableness of the fair value measurements for its other long-term investments, including obtaining and reviewing the audited annual financial statements of each hedge fund and private equity fund and periodically discussing each fund’s pricing with the fund manager. However, since the fund managers do not provide sufficient information to evaluate the pricing inputs and methods for each underlying investment, the inputs are considered to be unobservable. Accordingly, the fair values of White Mountains’s investments in hedge funds and private equity funds have been classified as Level 3 measurements. The fair value of White Mountains’s investments in hedge funds and private equity funds has been determined using net asset value. Prior to the liquidation of the Prospector Offshore Fund, in addition to the investments described above, White Mountains had $38.0 million of investment-related liabilities recorded at fair value and included in other liabilities as of December 31, 2014 . These liabilities relate to securities that have been sold short by limited partnerships in which White Mountains has investments and was required to consolidate under GAAP. These liabilities have a Level 1 designation. Fair Value Measurements by Level The following tables summarize White Mountains’s fair value measurements for investments as of June 30, 2015 and December 31, 2014 , by major security type and class by level. The major security types were based on the legal form of the securities, with a separate break-out for convertible fixed maturity investments as they may react similar to either debt securities or equity securities, depending on prevailing market conditions. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, foreign governments, municipalities or entities issuing asset-backed securities vary depending on the nature of the issuing entity type. White Mountains further disaggregates debt securities issued by corporations and equity securities by industry sector because investors often reference commonly used benchmarks and their subsectors to monitor risk and performance. Accordingly, White Mountains has further disaggregated these asset classes into subclasses based on the similar sectors and industry classifications it uses to evaluate investment risk and performance against commonly used benchmarks, such as the Barclays Intermediate Aggregate and S&P 500 indices: June 30, 2015 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 183.6 $ 131.5 $ 52.1 $ — Debt securities issued by corporations: Consumer 544.0 — 544.0 — Financials 405.0 — 405.0 — Health Care 306.2 — 306.2 — Industrial 281.3 — 281.3 — Communications 237.2 — 237.2 — Energy 194.2 — 194.2 — Utilities 149.0 — 149.0 — Technology 90.3 — 90.3 — Materials 98.4 — 98.4 — Other 5.5 — 5.5 — Total debt securities issued by corporations 2,311.1 — 2,311.1 — Municipal obligations 117.7 — 117.7 — Mortgage-backed and asset-backed securities 2,077.4 — 2,070.0 7.4 Foreign government, agency and provincial obligations 98.7 18.8 79.9 — Preferred stocks 86.0 — 14.8 71.2 Total fixed maturity investments 4,874.5 150.3 4,645.6 78.6 Short-term investments 840.5 838.3 2.2 — Common equity securities: Exchange traded funds (2) 361.5 340.1 21.4 — Financials 93.7 60.6 — 33.1 Consumer 51.6 51.6 — — Health Care 25.8 25.8 — — Industrial 32.3 32.3 — — Technology 38.2 38.2 — — Communications 23.8 23.8 — — Energy 2.5 2.5 — — Materials 3.6 3.6 — — Utilities 1.1 1.1 — — Other 79.7 — 79.7 — Total common equity securities 713.8 579.6 101.1 33.1 Convertible fixed maturity and preferred investments 4.5 — — 4.5 Other long-term investments (1) 370.6 — — 370.6 Total investments $ 6,803.9 $ 1,568.2 $ 4,748.9 $ 486.8 (1) Excludes carrying value of $3.9 associated with other long-term investment limited partnerships accounted for using the equity method. Excludes carrying value of $15.8 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method and $ (0.2) related to forward contracts. (2) ETFs traded on foreign exchanges are priced with an adjusted NAV and are therefore included within level 2 measurements. December 31, 2014 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 188.1 $ 134.1 $ 54.0 $ — Debt securities issued by corporations: Consumer 575.3 — 575.3 — Financials 431.4 — 431.4 — Health Care 284.2 — 284.2 — Industrial 225.4 — 219.8 5.6 Communications 214.3 — 214.3 — Energy 177.9 — 177.9 — Utilities 173.6 — 173.6 — Technology 118.0 — 118.0 — Materials 103.0 — 103.0 — Other 8.1 — 8.1 — Total debt securities issued by corporations 2,311.2 — 2,305.6 5.6 Municipal obligations 83.2 — 83.2 — Mortgage-backed and asset-backed securities 1,840.9 — 1,840.9 — Foreign government, agency and provincial obligations 275.1 21.3 253.8 — Preferred stocks 85.8 — 14.7 71.1 Total fixed maturity investments 4,784.3 155.4 4,552.2 76.7 Short-term investments 871.7 868.8 2.9 — Common equity securities: Financials 237.5 197.3 — 40.2 Consumer 161.2 161.1 .1 — Health Care 101.9 101.9 — — Industrial 67.5 67.5 — — Technology 54.3 54.3 — — Communications 32.9 32.9 — — Energy 32.6 32.6 — — Materials 21.2 21.2 — — Utilities 9.5 9.4 .1 — Other 83.0 9.7 73.3 — Total common equity securities 801.6 687.9 73.5 40.2 Convertible fixed maturity and preferred investments 20.5 — 12.3 8.2 Other long-term investments (1) 385.0 — — 385.0 Total investments $ 6,863.1 $ 1,712.1 $ 4,640.9 $ 510.1 (1) Excludes carrying value of $5.2 associated with other long-term investment limited partnerships accounted for using the equity method. Excludes carrying value of $16.8 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method. Debt securities issued by corporations The following table summarizes the ratings of the corporate debt securities held in White Mountains’s investment portfolio as of June 30, 2015 and December 31, 2014 : Fair Value at Millions June 30, 2015 December 31, 2014 AAA $ — $ — AA 200.5 236.9 A 887.6 957.8 BBB 1,213.5 1,105.9 BB 6.4 — Other 3.1 10.6 Debt securities issued by corporations (1) $ 2,311.1 $ 2,311.2 (1) Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor's Financial Services LLC (“Standard & Poor’s”) and 2) Moody’s Investor Service (“Moody's”). Mortgage-backed, Asset-backed Securities White Mountains purchases commercial and residential mortgage-backed securities with the goal of maximizing risk adjusted returns in the context of a diversified portfolio. White Mountains categorizes mortgage-backed securities as “non-prime” (also called “Alt A” or “A-”) if they are backed by collateral that has overall credit quality between prime and sub-prime based on White Mountains’s review of the characteristics of their underlying mortgage loan pools, such as credit scores and financial ratios. White Mountains considers sub-prime mortgage-backed securities as those that have underlying loan pools that exhibit weak credit characteristics, or those that are issued from dedicated sub-prime shelves or dedicated second-lien shelf registrations (i.e., White Mountains considers investments backed primarily by second-liens to be sub-prime risks regardless of credit scores or other metrics). The following table summarizes mortgage and asset-backed securities as of June 30, 2015 and December 31, 2014 : June 30, 2015 December 31, 2014 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: GNMA $ 355.4 $ 355.4 $ — $ 411.2 $ 411.2 $ — FNMA 38.9 38.9 — 36.6 36.6 — FHLMC 34.8 34.8 — 49.6 49.6 — Total Agency (1) 429.1 429.1 —

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	6 Months Ended
	Jun. 30, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets White Mountains has recognized goodwill and other identifiable intangible assets at the acquisition date fair values in connection with its purchases of subsidiaries. On February 23, 2015, Wobi acquired 56.2% of the outstanding share capital of Tnuva Finansit Ltd. (“Cashboard”) for NIS 9.5 million (approximately $2.4 million based upon the foreign exchange spot rate at the date of acquisition), representing a controlling financial interest. As of the acquisition date, Wobi recognized total assets acquired of $5.5 million , including $0.3 million of goodwill and $2.8 million of other intangible assets; and total liabilities assumed of $1.2 million at their estimated fair values. The following table shows the change in goodwill and other intangible assets: Three Months Ended June 30, Millions 2015 2014 Goodwill Other intangible assets Goodwill Other intangible assets Beginning balance $ 169.2 $ 193.4 $ 23.8 $ 62.4 Acquisitions of businesses — — — — Dispositions of businesses — — — — Amortization, including foreign currency translation — (7.0 ) — (2.6 ) Ending balance $ 169.2 $ 186.4 $ 23.8 $ 59.8 Six Months Ended June 30, Millions 2015 2014 Goodwill Other intangible assets Goodwill Other intangible assets Beginning balance $ 168.9 $ 197.5 $ — $ 20.7 Acquisitions of businesses .3 2.8 23.8 42.4 Dispositions of businesses — — — (.4 ) Amortization, including foreign currency translation — (13.9 ) — (2.9 ) Ending balance $ 169.2 $ 186.4 $ 23.8 $ 59.8

Debt

Debt	6 Months Ended
	Jun. 30, 2015
Debt Disclosure [Abstract]
Debt	Debt and Standby Letter of Credit Facilities White Mountains’s debt outstanding as of June 30, 2015 and December 31, 2014 consisted of the following: Millions June 30, December 31, 2012 OBH Senior Notes, at face value $ 275.0 $ 275.0 Unamortized original issue discount (.3 ) (.3 ) 2012 OBH Senior Notes, carrying value 274.7 274.7 SIG Senior Notes, at face value 400.0 400.0 Unamortized original issue discount (.3 ) (.3 ) SIG Senior Notes, carrying value 399.7 399.7 WTM Bank Facility — — Tranzact Bank Facility 70.6 68.7 Unamortized issuance cost (1.3 ) (1.3 ) Tranzact Bank Facility, carrying value 69.3 67.4 Other debt 4.8 4.8 Total debt $ 748.5 $ 746.6 WTM Bank Facility On August 14, 2013, White Mountains entered into a revolving credit facility with a syndicate of lenders administered by Wells Fargo Bank, N.A., which has a total commitment of $425.0 million and has a maturity date of August 14, 2018 (the “WTM Bank Facility”). In June 2015, White Mountains borrowed and repaid a total of $15.0 million under the WTM Bank Facility at a blended interest rate of 3.65% . As of June 30, 2015 , the WTM Bank Facility was undrawn. The WTM Bank Facility contains various affirmative, negative and financial covenants which White Mountains considers to be customary for such borrowings, including certain minimum net worth and maximum debt to capitalization standards. Tranzact Bank Facility On October 10, 2014, Tranzact entered into a secured credit facility with a syndicate of lenders administered by the PrivateBank and Trust Company (the “Tranzact Bank Facility”). The term of the credit facility ends on October 10, 2019. The Tranzact Bank Facility has a total commitment of $82.0 million , which consists of a $70.0 million term loan facility and a $12.0 million revolving facility. During the first six months ended June 30, 2015, Tranzact borrowed $4.5 million under the revolving facility and during the three and six months ended June 30, 2015 , Tranzact repaid a total of $1.3 million and $2.6 million under the term loan portion. As of June 30, 2015, the total amount outstanding under the Tranzact Bank Facility was $70.6 million . The Tranzact Bank Facility, which is secured by intellectual property and the common stock of Tranzact and its subsidiaries, contains various affirmative, negative and financial covenants which White Mountains considers to be customary for such borrowings, including a minimum fixed charge coverage ratio and a maximum leverage ratio. Stand By Letter of Credit Facilities On November 25, 2014, Sirius International entered into two stand by letter of credit facility agreements totaling $200.0 million to provide capital support for its Lloyds Syndicate 1945. One letter of credit is a $125.0 million facility from Nordea Bank Finland plc (the “Nordea facility”), $100.0 million of which is issued on an unsecured basis. The second letter of credit is a $75.0 million facility with Lloyds Bank plc (the “Lloyds Bank facility”), $25.0 million of which is issued on an unsecured basis. The Nordea facility and the Lloyds Bank facility are renewable annually. The unsecured portions of the Nordea facility and the Lloyds Bank facility are subject to various affirmative, negative and financial covenants that White Mountains considers to be customary for such borrowings, including certain minimum net worth and maximum debt to capitalization standards. Sirius International has other secured letter of credit and trust arrangements with various financial institutions to support its insurance operations. Debt and Standby Letter of Credit Facility Covenants At June 30, 2015 , White Mountains was in compliance with all of the covenants under the WTM Bank Facility, the OneBeacon U.S. Holdings, Inc. (“OBH”) Senior Notes, the Sirius International Group, Ltd. (“SIG”) Senior Notes, the Tranzact Bank Facility, the Nordea facility and the Lloyd’s Bank facility.

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2015
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company and its Bermuda domiciled subsidiaries are not subject to Bermuda income tax under current Bermuda law. In the event there is a change in the current law such that taxes are imposed, the Company and its Bermuda domiciled subsidiaries would be exempt from such tax until March 31, 2035, pursuant to the Bermuda Exempted Undertakings Tax Protection Act of 1966. The Company has subsidiaries and branches that operate in various other jurisdictions around the world that are subject to tax in the jurisdictions in which they operate. The jurisdictions in which the Company’s consolidated subsidiaries and branches are subject to tax are Australia, Belgium, Canada, Denmark, Germany, Gibraltar, Israel, Luxembourg, Malaysia, the Netherlands, Peru, Singapore, Sweden, Switzerland, the United Kingdom and the United States. White Mountains’s income tax benefit for the three months ended June 30, 2015 represented an effective tax rate of 46.6% . White Mountains’s income tax expense for the six months ended for June 30, 2015 represented an effective tax rate of 27.5% . White Mountains’s income tax expense for the three and six months ended for June 30, 2014 represented effective tax rates of 22.4% and 25.0% . The effective tax rate for the three months ended June 30, 2015 was higher than the U.S. statutory rate of 35% primarily due to changes in forecasted earnings by jurisdiction. The effective tax rates for the six months ended June 30, 2015 and the three and six months ended 2014 were lower than the U.S. statutory rate of 35% due to income generated in jurisdictions with lower tax rates than the United States. In arriving at the effective tax rate for the three and six months ended June 30, 2015 and 2014 , White Mountains forecasted all income and expense items including the change in unrealized investment gains (losses) and realized investment gains (losses) for the years ending December 31, 2015 and 2014. White Mountains records a valuation allowance against deferred tax assets if it becomes more likely than not that all or a portion of a deferred tax asset will not be realized. Changes in valuation allowances from period to period are included in income tax expense in the period of change. In determining whether or not a valuation allowance, or change therein, is warranted, White Mountains considers factors such as prior earnings history, expected future earnings, carryback and carryforward periods and strategies that if executed would result in the realization of a deferred tax asset. It is possible that certain planning strategies or projected earnings in certain subsidiaries may not be feasible to utilize the entire deferred tax asset, which could result in material changes to White Mountains’s deferred tax assets and tax expense. White Mountains is no longer subject to U.S. federal or state tax examinations by tax authorities for years before 2007. With few exceptions, White Mountains is no longer subject to non-U.S. income tax examinations by tax authorities for years before 2005. On July 28, 2011, the IRS commenced an examination of the income tax returns for 2007, 2008 and 2009 for certain U.S. subsidiaries of OneBeacon. On July 17, 2013, OneBeacon received a revised Form 4549-A (Income Tax Discrepancy Adjustments) from the IRS relating to the examination of tax years 2007, 2008 and 2009. The estimated total assessment, including interest, utilization of alternative minimum and foreign tax credit carryovers and capital loss carrybacks, is $74.8 million . However, $60.2 million of the proposed adjustments relate to items for which the expense deduction has been disallowed in a year being examined, but ultimate deductibility is highly certain to occur in a later period. Because of the impact of deferred tax accounting, other than interest and penalties, the disallowance of these deductions in the exam period would not affect the effective tax rate, but would accelerate the payment of cash to the taxing authority. White Mountains disagrees with the adjustments proposed by the IRS and is defending its position. Although the timing of the resolution of these issues is uncertain, it is reasonably possible that the resolution could occur within the next twelve months. White Mountains does not expect the resolution of this examination to result in a material adverse change to its financial position results of operations and cash flows. On September 5, 2013, the IRS commenced an examination of the income tax returns for 2010, 2011 and 2012 for certain U.S. subsidiaries of OneBeacon. White Mountains does not expect the resolution of this examination to result in a material change to its financial position, results of operations and cash flows. On December 18, 2014, the IRS commenced an examination of the 2012 income tax return for Guilford Holdings, Inc. and subsidiaries. White Mountains does not expect the resolution of this examination to result in a material change to its financial position, results of operations and cash flows.

Derivatives

Derivatives	6 Months Ended
	Jun. 30, 2015
Derivatives	Derivatives
Variable Annuity
Derivatives	Variable Annuity Reinsurance White Mountains has entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan. At June 30, 2015 and December 31, 2014 , the total guarantee value was approximately ¥ 100.7 billion (approximately $0.8 billion at exchange rates on that date) and ¥134.2 billion (approximately $1.1 billion at exchange rates on that date), respectively. The collective account values of the underlying variable annuities were approximately 113% of the guarantee value at both June 30, 2015 and December 31, 2014 . During the second quarter of 2015, the variable annuity contracts reinsured by WM Life Re began to mature. WMLife Re is in runoff, an all of its contracts will mature by June 30, 2016. The following table summarizes the pre-tax operating results of WM Life Re for the three and six months ended June 30, 2015 and 2014 . Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Fees, included in other revenue $ 2.6 $ 5.1 $ 5.6 $ 10.5 Change in fair value of variable annuity liability, included in other revenue .7 22.0 1.1 20.7 Change in fair value of derivatives, included in other revenue (2.3 ) (26.2 ) (7.8 ) (32.7 ) Foreign exchange, included in other revenue — .2 (1.2 ) .5 Other investment income and (losses) gains (.1 ) .2 (.2 ) .5 Total revenue .9 1.3 (2.5 ) (.5 ) Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses — .3 — .3 Death benefit claims paid, included in general and administrative expenses (.1 ) (.1 ) — (.1 ) General and administrative expenses (.9 ) (1.0 ) (2.3 ) (2.4 ) Pre-tax loss $ (.1 ) $ .5 $ (4.8 ) $ (2.7 ) The following summarizes realized and unrealized derivative gains (losses) recognized in other revenue for the three and six months ended June 30, 2015 and 2014 and the carrying values, included in other assets, at June 30, 2015 and December 31, 2014 by type of instrument: Gains (losses) Carrying Value Three Months Ended Six Months Ended As of June 30, June 30, June 30, December 31, 2014 Millions 2015 2014 2015 2014 Fixed income/interest rate $ (5.9 ) $ (5.6 ) $ 3.4 $ (12.0 ) $ (.1 ) $ (1.7 ) Foreign exchange 9.2 (6.4 ) (2.9 ) (12.9 ) 29.0 44.1 Equity (5.6 ) (14.2 ) (8.3 ) (7.8 ) 6.8 14.0 Total $ (2.3 ) $ (26.2 ) $ (7.8 ) $ (32.7 ) $ 35.7 $ 56.4 The following tables summarize the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the three and six months ended June 30, 2015 and 2014 : Three Months Ended June 30, 2015 Variable Annuity Assets Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ 1.1 $ 17.0 $ 31.2 $ (1.2 ) $ 47.0 Purchases — — — — — Realized and unrealized gains (losses) .7 (8.3 ) .6 5.4 (2.3 ) Transfers in — — — — — Sales/settlements — (.4 ) .7 (9.3 ) (9.0 ) End of period $ 1.8 $ 8.3 $ 32.5 $ (5.1 ) $ 35.7 Six Months Ended June 30, 2015 Variable Annuity Assets Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ .7 $ 18.9 $ 33.8 $ 3.7 $ 56.4 Purchases — — — — — Realized and unrealized gains (losses) 1.1 (9.0 ) (5.9 ) 7.1 (7.8 ) Transfers in — — — — — Sales/settlements — (1.6 ) 4.6 (15.9 ) (12.9 ) End of period $ 1.8 $ 8.3 $ 32.5 $ (5.1 ) $ 35.7 Three Months Ended June 30, 2014 Variable Annuity Assets Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ (54.1 ) $ 64.1 $ 25.1 $ .4 $ 89.6 Purchases — — — — — Realized and unrealized gains (losses) 22.3 (10.6 ) (15.5 ) (.1 ) (26.2 ) Transfers in — — — — — Sales/settlements — (.2 ) 12.7 (1.1 ) 11.4 End of period $ (31.8 ) $ 53.3 $ 22.3 $ (.8 ) $ 74.8 Six Months Ended June 30, 2014 Variable Annuity (Liabilities) Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ (52.8 ) $ 63.4 $ 4.7 $ 1.1 $ 69.2 Purchases — — — — — Realized and unrealized gains (losses) 21.0 (9.9 ) (22.5 ) (.3 ) (32.7 ) Transfers in — — — — — Sales/settlements — (.2 ) 40.1 (1.6 ) 38.3 End of period $ (31.8 ) $ 53.3 $ 22.3 $ (.8 ) $ 74.8 (1) Consists of over-the-counter instruments. (2) Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available. Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index. (3) Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded. In addition to derivative instruments, WM Life Re held cash and fixed maturity investments posted as collateral to its variable annuity reinsurance counterparties. The total collateral includes the following: Millions June 30, 2015 December 31, 2014 June 30, 2014 Cash $ 20.9 $ 23.7 $ 27.8 Fixed maturity investments 4.4 9.5 14.5 Total $ 25.3 $ 33.2 $ 42.3 Collateral in the form of fixed maturity securities consists of Government of Japan Bonds, which are recorded at fair value. Collateral in the form of short-term investments consists of money-market instruments, carried at amortized cost, which approximates fair value. All of White Mountains’s variable annuity reinsurance liabilities were classified as Level 3 measurements at June 30, 2015 and 2014 . The fair value of White Mountains’s variable annuity reinsurance liabilities are estimated using actuarial and capital market assumptions related to the projected discounted cash flows over the term of the reinsurance agreement. Actuarial assumptions regarding future policyholder behavior, including surrender and lapse rates, are generally unobservable inputs and significantly impact the fair value estimates. Market conditions including, but not limited to, changes in interest rates, equity indices, market volatility and foreign currency exchange rates as well as the variations in actuarial assumptions regarding policyholder behavior may result in significant fluctuations in the fair value estimates. Generally, the liabilities associated with these guarantees increase with declines in the equity markets, interest rates and currencies against the Japanese yen, as well as with increases in market volatilities. White Mountains uses derivative instruments, including put options, interest rate swaps, total return swaps on bond and equity indices and forwards and futures contracts on major equity indices, currency pairs and government bonds, to mitigate the risks associated with changes in the fair value of the reinsured variable annuity guarantees. The types of inputs used to estimate the fair value of these derivative instruments, with the exception of actuarial assumptions regarding policyholder behavior and risk margins, are generally the same as those used to estimate the fair value of variable annuity liabilities. The following summarizes quantitative information about significant unobservable inputs associated with the fair value estimates for variable annuity reinsurance liabilities and derivative instruments that have been classified as Level 3 measurements: ($ in Millions) June 30, 2015 Description Fair Value Valuation Technique(s) Unobservable Input Range Weighted Average Variable annuity benefit guarantee liabilities $ (1.8 ) Discounted cash flows Surrenders 0-1 year 0.1 % - 40.0% 40.0 % Mortality 0.0 % - 6.4% 1.1 % Foreign exchange volatilities 0-1 year 10.6 % - 16.1% 12.8 % Index volatilities 0-1 year 24.5 % - 27.9% 26.3 % Foreign exchange options $ 2.7 Counterparty valuations, adjusted for unwind quote discount Adjustment to counterparty valuations 0.5 % - 11.6% 5.3 % Equity index options $ 5.6 Counterparty valuations, adjusted for unwind quote discount Adjustment to counterparty valuations (0.7 )% - 6.5% 1.5 % WM Life Re enters into both over-the-counter (“OTC”) and exchange traded derivative instruments to economically hedge the liability from the variable annuity benefit guarantee. In the case of OTC derivatives, WM Life Re has exposure to credit risk for amounts that are uncollateralized by counterparties. WM Life Re’s internal risk management guidelines establish net counterparty exposure thresholds that take into account OTC counterparties’ credit ratings. The OTC derivative contracts are subject to restrictions on liquidation of the instruments and distribution of proceeds under collateral agreements. In the case of exchange traded instruments, WM Life Re has exposure to credit risk for amounts uncollateralized by margin balances. WM Life Re has entered into master netting agreements with certain of its counterparties whereby the collateral provided (held) is calculated on a net basis. The following summarizes amounts offset under master netting agreements: June 30, 2015 December 31, 2014 Millions Gross asset amounts before offsets (1) Gross liability amounts offset under master netting arrangements Net amounts recognized in Other Assets Gross asset amounts before offsets (1) Gross liability amounts offset under master netting arrangements Net amounts recognized in Other Assets Interest rate contracts OTC $ 2.2 $ (2.1 ) $ 0.1 $ 1.0 $ (5.4 ) $ (4.4 ) Exchange traded .2 (.6 ) (.4 ) 2.8 (.1 ) 2.7 Foreign exchange contracts OTC 31.6 — 31.6 45.5 — 45.5 Exchange traded — (2.3 ) (2.3 ) — (1.4 ) (1.4 ) Equity contracts OTC 10.2 (1.2 ) 9.0 11.7 (.2 ) 11.5 Exchange traded — (2.2 ) (2.2 ) 3.4 (.9 ) 2.5 Total (2) $ 44.2 $ (8.4 ) $ 35.8 $ 64.4 $ (8.0 ) $ 56.4 (1) Amount equal to fair value of instrument as recognized in other assets (2) All derivative instruments held by WM Life Re are subject to master netting arrangements. The following summarizes the value, collateral held or provided by WM Life Re and net exposure to credit losses on OTC and exchange traded derivative instruments by counterparty recorded within other assets: June 30, 2015 Millions Net amount of assets reflected in Balance Sheet Collateral provided to counterparty - Cash Collateral provided to counter-party - Financial Instruments Net amount of exposure after effect of collateral provided Excess collateral provided to counter-party- Cash Excess collateral provided - Financial Instruments Counter-party collateral held by WMLife Re - Cash Net amount of exposure to counter-party Standard & Poor's Rating (1) Bank of America $ 2.0 $ — $ — $ 2.0 $ — $ — $ — $ 2.0 A Barclays — — — — — — — — A JP Morgan 16.8 — — 16.8 — — 6.5 10.3 A + Royal Bank of Scotland 1.0 — — 1.0 — — — 1.0 A - Nomura (.3 ) .3 — — 4.9 4.4 — 9.3 BBB + Citigroup - OTC 21.3 — — 21.3 — — 4.9 16.4 A Citigroup - Exchange Traded (5.0 ) 5.0 — — 10.7 — — 10.7 A Total $ 35.8 $ 5.3 $ — $ 41.1 $ 15.6 $ 4.4 $ 11.4 $ 49.7 December 31, 2014 Millions Net amount of assets reflected in Balance Sheet Collateral provided to counter-party - Cash Collateral provided to counter-party - Financial Instruments Net amount of exposure after effect of collateral provided Excess collateral provided to counter-party- Cash Excess collateral provided - Financial Instruments Counter-party collateral held by WMLife Re- Cash Net amount of exposure to counter-party Standard & Poor's Rating (1) Bank of America $ 5.6 $ — $ — $ 5.6 $ — $ — $ — $ 5.6 A Barclays .1 — — .1 — — — .1 A JP Morgan 24.3 — — 24.3 — — 8.8 15.5 A + Royal Bank of Scotland 4.0 — — 4.0 — — — 4.0 A Nomura (3.5 ) 3.5 — — 1.7 9.5 — 11.2 BBB + Citigroup - OTC 22.2 — — 22.2 — — 1.1 21.1 A Citigroup - Exchange Traded 3.7 — — 3.7 16.0 — — 19.7 A Total $ 56.4 $ 3.5 $ — $ 59.9 $ 17.7 $ 9.5 $ 9.9 $ 77.2 (1) Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-three creditworthiness ratings), “A” (Strong, which is the sixth highest of twenty-three creditworthiness ratings), “A-” (Strong, which is the seventh highest of twenty-three creditworthiness ratings) and “BBB+” (Adequate, which is the eighth highest of twenty-three creditworthiness ratings).
Forward Contracts
Derivatives	Forward Contracts White Mountains has entered into currency forward contracts at Sirius Group. White Mountains monitors its exposure to foreign currency and adjusts its forward positions within the risk guidelines and ranges established by senior management for each currency, as necessary. While White Mountains actively manages its forward positions, mismatches between movements in foreign currency rates and its forward contracts may result in currency positions being outside the pre-defined ranges and/or foreign currency losses. At June 30, 2015 , White Mountains held approximately $31.9 million (SEK 262.7 million ) total gross notional value of foreign currency forward contracts. All of White Mountains’s forward contracts are traded over-the-counter. The fair value of the contracts has been estimated using OTC quotes for similar instruments and accordingly, the measurements have been classified as Level 2 measurements at June 30, 2015 . The net realized and unrealized derivative losses recognized in net realized and unrealized investment gains (losses) for both the three and six months ended June 30, 2015 was $ 0.4 million. The net realized and unrealized derivative losses recognized in net realized and unrealized investment gains (losses) for the three and six months ended June 30, 2014 was break-even million and $(0.1) million . All of White Mountains’s forward contracts are subject to master netting agreements. As of June 30, 2015 and December 31, 2014, the gross liability amount offset under master netting arrangements and the net amount recognized in other investments approximately offset each other. White Mountains does not hold or provide any collateral for the forward contracts. The following table summarizes the notional amounts and uncollateralized balances associated with forward currency contracts: June 30, 2015 December 31, 2014 Millions Notional Amount Carrying Value Standard & Poor's Rating (1) Notional Amount Carrying Value Barclays Bank Plc $ .8 $ — A- $ 2.1 $ — Deutsche Bank 8.5 — BBB+ — — Goldman Sachs 5.0 (.1 ) AA- 8.7 — HSBC Bank Plc 12.0 (.1 ) AA- 11.2 — JP Morgan 1.2 — A 5.7 — Royal Bank of Canada 4.4 — AA- 5.4 — Total $ 31.9 $ (.2 ) $ 33.1 $ — (1) Standard & Poor's ratings as detailed above are: “AA-” (Very Strong, which is the sixth highest of twenty-three creditworthiness ratings), “A+” (Strong, which is the seventh highest of twenty-three creditworthiness ratings) and “A” (Strong, which is the eighth highest of twenty-three creditworthiness ratings).
Interest Rate Cap
Derivatives	Interest Rate Cap In May 2007, Sirius International Group, Ltd. (“SIG”), an intermediate holding company of Sirius Group, issued the SIG Preference Shares, with an initial fixed annual dividend rate of 7.506% . In June 2017, the fixed rate will move to a floating rate equal to the greater of (i) 7.506% and (ii) 3 -month LIBOR plus 320 basis points. In July 2013, SIG executed the Interest Rate Cap for the period from June 2017 to June 2022 to protect against a significant increase in interest rates during that 5 -year period. The Interest Rate Cap economically fixes the annual dividend rate on the SIG Preference Shares from June 2017 to June 2022 at 8.30% . The cost of the Interest Rate Cap was an upfront premium of 395 basis points of the $250.0 million notional value, or approximately $9.9 million for the full notional amount. The Interest Rate Cap does not qualify for hedge accounting. It is recorded in other assets at fair value. Changes in fair value are recognized within other revenue. Collateral held is recorded within short-term investments with an equal amount recognized as a liability to return collateral. The fair value of the Interest Rate Cap has been estimated using a single broker quote and accordingly, has been classified as a Level 3 measurement at June 30, 2015 . The following tables summarize the changes in the fair value of the Interest Rate Cap for the three and six months ended June 30, 2015 and 2014: Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Beginning of period $ 3.1 $ 8.2 $ 4.1 $ 11.1 Net realized and unrealized gains (losses) .2 (2.0 ) (.8 ) (4.9 ) End of period $ 3.3 $ 6.2 $ 3.3 $ 6.2 White Mountains does not provide any collateral to the interest rate counterparties. Under the terms of the Interest Rate Cap, White Mountains holds collateral in respect of future amounts due. White Mountains’s liability to return that collateral is based on the amounts provided by the counterparty and investment earnings thereon. The following table summarizes the Interest Rate Cap collateral balances held by White Mountains and ratings by counterparty: June 30, 2015 Millions Collateral Balances Held Standard & Poor’s Rating (1) Barclays Bank Plc $ 2.3 A- Nordea Bank Finland Plc 1.0 AA- Total $ 3.3 (1) Standard & Poor’s ratings as detailed above are: “AA-” (Very Strong, which is the fourth highest of twenty-three creditworthiness ratings) and “A-” (Strong, which is the seventh highest of twenty-three creditworthiness ratings).
Weather derivatives
Derivatives	Weather Derivatives For the three and six months ended June 30, 2015 , Sirius Group recognized $2.3 million and $3.1 million of net gains on its weather and weather contingent derivatives portfolio. For the three and six months ended June 30, 2014, Sirius Group recognized $ 1.0 million and $0.6 million of net losses on its weather and weather contingent derivatives portfolio. The fair values of the assumed contracts are subject to change in the near-term and reflect management’s best estimate based on various factors including, but not limited to, observed and forecasted weather conditions, changes in interest or foreign currency exchange rates and other market factors. Estimating the fair value of derivative instruments that do not have quoted market prices requires management’s judgment in determining amounts that could reasonably be expected to be received from or paid to a third party to settle the contracts. Such amounts could be materially different from the amounts that might be realized in an actual transaction to settle the contract with a third party. Because of the significance of the unobservable inputs used to estimate the fair value of Sirius Group’s weather risk contracts, the fair value measurements of the contracts are deemed to be Level 3 measurements in the fair value hierarchy.
Tranzact Interest Rate Swap
Derivatives	Tranzact Interest Rate Swap Tranzact has a $70.0 million term loan facility that carries a variable rate equal to the U.S. dollar LIBOR rate, plus an applicable margin. At June 30, 2015, the variable interest rate on the term loan was 4.185% , including a margin over LIBOR of 4.0% . Effective October 10, 2014, to effectively fix the rate it pays on this term loan, Tranzact entered into an interest rate swap agreement with a notional amount of $70.0 million at inception, which decreases over the term of the swap by amounts equivalent to scheduled principal repayments made on Tranzact’s term loan. As of June 30, 2015, the notional amount was $66.1 million . Under the terms of the swap agreement, Tranzact pays a fixed rate of 1.34% and receives a variable rate, which is reset monthly, based on the then-current U.S. dollar LIBOR rate. The variable rate received by Tranzact under the swap agreement was 0.1518% at inception and 0.1866% at June 30, 2015. The total current effective rate on Tranzact's debt was 5.5% at June 30, 2015. The swap is measured at fair value with changes therein recognized within other revenues and is accounted for as a non-hedge derivative instrument. As of June 30, 2015, the estimated fair value of the swap was $(0.2) million . There are no collateral arrangements associated with the swap.

Municipal Bond Guarantee

Municipal Bond Guarantee	6 Months Ended
	Jun. 30, 2015
Guarantees [Abstract]
Municipal Bond Guarantee Insurance	Municipal Bond Guarantee Insurance In 2012, HG Global was capitalized with $594.5 million from White Mountains and $14.5 million from non-controlling interests to fund BAM, a newly formed mutual municipal bond insurer. As of June 30, 2015 , White Mountains owned 96.9% of HG Global’s preferred equity and 88.4% of its common equity. HG Global, together with its subsidiaries, provided the initial capitalization of BAM through the purchase of $503.0 million of BAM surplus notes. Through HG Re, which had statutory capital of $454.6 million at June 30, 2015 , HG Global provides first loss reinsurance protection for policies underwritten by BAM of up to 15% of par outstanding, on a per policy basis. HG Re’s obligations to BAM are collateralized in trusts, and there is an aggregate loss limit that is equal to the total assets in the collateral trusts at any point in time. For the three and six months ended June 30, 2015 , HG Global had pre-tax income of $3.1 million and $8.6 million , which included $3.9 million and $7.9 million of interest income on the BAM surplus notes. For the three and six months ended June 30, 2014, HG Global had pre-tax income of $5.0 million and $9.6 million , which included $4.0 million and $7.9 million of interest income on the BAM surplus notes. For the three and six months ended June 30, 2015 , White Mountains reported pre-tax losses of $14.1 million and $22.9 million on BAM that were recorded in net loss attributable to non-controlling interests, which included $3.9 million and $7.9 million of interest expense on the BAM surplus notes. For the three and six months ended June 30, 2014, White Mountains reported pre-tax losses of $8.0 million and $16.6 million on BAM that were recorded in net loss attributable to non-controlling interests, which included $4.0 million and $7.9 million of interest expense on the BAM surplus notes. Effective January 1, 2014, HG Global and BAM agreed to change the interest rate on the BAM surplus notes for the five years ending December 31, 2018 from a fixed rate of 8% to a variable rate equal to the one -year U.S. treasury rate plus 300 basis points, set annually, which is 3.15% and 3.13% for 2015 and 2014. Prior to the end of 2018, BAM has the option to extend the variable rate period for an additional three years. At the end of the variable rate period, the interest rate will be fixed at the higher of the then current variable rate or 8% . BAM is required to seek regulatory approval to pay interest and principal on its surplus notes only when adequate capital resources have accumulated beyond BAM’s initial capitalization and a level that continues to support its outstanding obligations, business plan and ratings. All of the contracts issued by BAM are accounted for as insurance contracts under ASC 944-605, Financial Guarantee Insurance Contracts. Premiums are received upfront and an unearned premium revenue liability, equal to the amount of the cash received, is established at contract inception. Premium revenues are recognized in revenue over the period of the contracts in proportion to the amount of insurance protection provided using a constant rate. The constant rate is calculated based on the relationship between the par outstanding in a given reporting period compared with the sum of each of the par amounts outstanding for all periods. The following table provides a schedule of BAM’s insured obligations: June 30, 2015 December 31, 2014 Contracts outstanding 2,436 1,750 Remaining weighted average contract period outstanding (in years) 12.7 12.8 Contractual debt service outstanding (in millions): Par $ 17,766.6 $ 12,362.5 Interest 9,588.1 7,086.9 Total debt service outstanding $ 27,354.7 $ 19,449.4 Gross unearned insurance premiums $ 37.1 $ 27.6

Earnings Per Share

Earnings Per Share	6 Months Ended
	Jun. 30, 2015
Earnings Per Share [Abstract]
Earnings Per Share	Earnings Per Share White Mountains calculates earnings per share on the two-class method and accordingly, the net income allocable to common shareholders, undistributed net earnings and weighted average number of common shares outstanding exclude amounts associated with restricted shares that are considered participating securities. Diluted earnings per share amounts are based on the weighted average number of common shares, net of the effect of unvested restricted shares and potentially dilutive common shares outstanding. The following table outlines the Company’s computation of earnings per share from continuing operations for the three and six months ended June 30, 2015 and 2014 . (See Note 17 - “Discontinued Operations” ). Three Months Ended Six Months Ended June 30, June 30, 2015 2014 2015 2014 Basic and diluted earnings per share numerators (in millions): Net income from continuing operations attributable to White Mountains’s common shareholders $ 4.6 $ 92.9 $ 81.0 $ 188.9 Allocation of income for unvested restricted common shares (.1 ) (1.2 ) (.9 ) (2.3 ) Dividends declared on participating restricted common shares (1) — — (.1 ) (.1 ) Total allocation to restricted common shares (.1 ) (1.2 ) (1.0 ) (2.4 ) Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 4.5 $ 91.7 $ 80.0 $ 186.5 Undistributed net earnings (in millions): Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 4.5 $ 91.7 $ 80.0 $ 186.5 Dividends declared net of restricted common share amounts (1) — — (5.9 ) (6.1 ) Total undistributed net earnings, net of restricted common share amounts $ 4.5 $ 91.7 $ 74.1 $ 180.4 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 5,985.7 6,162.2 5,982.1 6,166.0 Average unvested restricted shares (2) (71.4 ) (81.3 ) (65.2 ) (75.9 ) Basic earnings per share denominator 5,914.3 6,080.9 5,916.9 6,090.1 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 5,985.7 6,162.2 5,982.1 6,166.0 Average unvested restricted common shares (2) (71.4 ) (81.3 ) (65.2 ) (75.9 ) Average outstanding dilutive options to acquire common shares (3) — — — — Diluted earnings per share denominator 5,914.3 6,080.9 5,916.9 6,090.1 Basic earnings per share (in dollars): Net income attributable to White Mountains’s common shareholders $ .76 $ 15.08 $ 13.53 $ 30.61 Dividends declared and paid — — (1.00 ) (1.00 ) Undistributed earnings $ .76 $ 15.08 $ 12.53 $ 29.61 Diluted earnings per share (in dollars): Net income attributable to White Mountains’s common shareholders $ .76 $ 15.08 $ 13.53 $ 30.61 Dividends declared and paid — — (1.00 ) (1.00 ) Undistributed earnings $ .76 $ 15.08 $ 12.53 $ 29.61 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (2) Restricted shares outstanding vest either in equal annual installments or upon a stated date. (See Note 15 - “Employee Share-Based Compensation Plans”) . (3) The diluted earnings per share denominator for the three and six months ended June 30, 2015 and 2014 does not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.

Non-controlling Interest

Non-controlling Interest	6 Months Ended
	Jun. 30, 2015
Noncontrolling Interest [Abstract]
Noncontrolling Interest Disclosure [Text Block]	The following table details the balance of non-controlling interests included in White Mountains’s total equity and the related percentage of each consolidated entity’s total equity owned by non-controlling shareholders as of June 30, 2015 and December 31, 2014: June 30, 2015 December 31, 2014 $ in millions Non-controlling Percentage Non-controlling Equity Non-controlling Percentage Non-controlling Equity OneBeacon Ltd. 24.7 % $ 255.0 24.7 % $ 258.4 SIG Preference Shares 100.0 250.0 100.0 250.0 Other, excluding mutuals and reciprocals HG Global 3.1 17.5 3.1 17.9 MediaAlpha 40.0 21.5 40.0 22.6 Tranzact 36.8 80.3 36.8 88.2 Wobi 4.7 2.3 36.7 5.4 Dewar 31.9 3.5 18.0 3.4 Prospector Offshore Fund — — 23.4 31.1 Total other, excluding mutuals and reciprocals 125.1 168.6 Mutuals and reciprocals BAM 100.0 (131.8 ) 100.0 (121.9 ) SSIE 100.0 (14.4 ) 100.0 (12.4 ) Total non-controlling interests $ 483.9 $ 542.7

Segment Information

Segment Information	6 Months Ended
	Jun. 30, 2015
Segment Reporting [Abstract]
Segment Informartion	Segment Information White Mountains has determined that its reportable segments are OneBeacon, Sirius Group, HG Global/BAM and Other Operations. White Mountains has made its segment determination based on consideration of the following criteria: (i) the nature of the business activities of each of the Company’s subsidiaries and affiliates; (ii) the manner in which the Company’s subsidiaries and affiliates are organized; (iii) the existence of primary managers responsible for specific subsidiaries and affiliates; and (iv) the organization of information provided to the chief operating decision makers and the Board of Directors. Significant intercompany transactions among White Mountains’s segments have been eliminated herein. Financial information for White Mountains’s segments follows: HG Global/BAM Millions OneBeacon Sirius Group HG BAM Other Operations Total Three Months Ended June 30, 2015 Earned insurance and reinsurance premiums $ 319.3 $ 215.5 $ .6 $ .1 $ 1.8 $ 537.3 Net investment income 10.1 9.3 .5 .8 3.1 23.8 Net investment income (loss) - surplus note interest — — 3.9 (3.9 ) — — Net realized and unrealized investment losses (14.0 ) (42.7 ) (1.4 ) (1.9 ) (1.1 ) (61.1 ) Other (loss) revenue (1.2 ) 5.4 — .2 83.1 87.5 Total revenues 314.2 187.5 3.6 (4.7 ) 86.9 587.5 Losses and LAE 194.5 92.1 — — 1.7 288.3 Insurance and reinsurance acquisition expenses 56.4 47.5 .1 .6 .7 105.3 Other underwriting expenses 52.9 27.7 — .1 — 80.7 General and administrative expenses 4.2 6.8 .4 8.7 88.0 108.1 Interest expense 3.3 6.6 — — 1.0 10.9 Total expenses 311.3 180.7 .5 9.4 91.4 593.3 Pre-tax income (loss) $ 2.9 $ 6.8 $ 3.1 $ (14.1 ) $ (4.5 ) $ (5.8 ) HG Global/BAM Millions OneBeacon Sirius Group HG BAM Other Operations Total Six Months Ended June 30, 2015 Earned insurance and reinsurance premiums $ 605.9 $ 420.5 $ 1.1 $ .3 $ 3.8 $ 1,031.6 Net investment income 20.6 17.3 .9 1.8 6.5 47.1 Net investment income (loss) - surplus note interest — — 7.9 (7.9 ) — — Net realized and unrealized investment gains (loses) .2 37.2 (.3 ) 1.1 4.1 42.3 Other revenue (loss) (4.3 ) (22.0 ) — .3 149.3 123.3 Total revenues 622.4 453.0 9.6 (4.4 ) 163.7 1,244.3 Losses and LAE 360.4 189.6 — — 3.7 553.7 Insurance and reinsurance acquisition expenses 107.4 90.1 .2 1.4 1.8 200.9 Other underwriting expenses 108.8 53.2 — .2 — 162.2 General and administrative expenses 8.3 13.0 .8 16.9 185.5 224.5 Interest expense 6.5 13.4 — — 2.1 22.0 Total expenses 591.4 359.3 1.0 18.5 193.1 1,163.3 Pre-tax income (loss) $ 31.0 $ 93.7 $ 8.6 $ (22.9 ) $ (29.4 ) $ 81.0 HG Global/BAM Millions OneBeacon Sirius Group HG BAM Other Operations Total Three Months Ended June 30, 2014 Earned insurance and reinsurance premiums $ 290.9 $ 209.6 $ .3 $ .1 $ 2.5 $ 503.4 Net investment income 12.2 11.8 .3 1.4 3.9 29.6 Net investment income (loss) - surplus note interest — — 4.0 (4.0 ) — — Net realized and unrealized investment gains 21.9 58.4 .9 3.8 28.9 113.9 Other revenue (loss) 1.0 (19.2 ) — .1 24.5 6.4 Total revenues 326.0 260.6 5.5 1.4 59.8 653.3 Losses and LAE 186.1 91.4 — — 5.2 282.7 Insurance and reinsurance acquisition expenses 49.6 48.8 .1 .5 (.1 ) 98.9 Other underwriting expenses 51.7 30.2 — .1 (.1 ) 81.9 General and administrative expenses 3.6 6.9 .4 8.8 50.6 70.3 Interest expense 3.3 6.6 — — .1 10.0 Total expenses 294.3 183.9 .5 9.4 55.7 543.8 Pre-tax income (loss) $ 31.7 $ 76.7 $ 5.0 $ (8.0 ) $ 4.1 $ 109.5 HG Global/BAM Millions OneBeacon Sirius Group HG BAM Other Operations Total Six Months Ended June 30, 2014 Earned insurance and reinsurance premiums $ 567.4 $ 425.1 $ .5 $ .2 $ 3.8 $ 997.0 Net investment income 22.6 20.5 .6 2.8 7.8 54.3 Net investment income (loss) - surplus note interest — — 7.9 (7.9 ) — — Net realized and unrealized investment gains 40.8 86.5 1.6 6.8 42.0 177.7 Other revenue (loss) 2.0 (24.5 ) — .3 25.2 3.0 Total revenues 632.8 507.6 10.6 2.2 78.8 1,232.0 Losses and LAE 335.5 170.2 — — 6.3 512.0 Insurance and reinsurance acquisition expenses 96.3 96.2 .2 .9 .4 194.0 Other underwriting expenses 101.1 62.0 — .2 — 163.3 General and administrative expenses 6.9 15.3 .8 17.7 79.9 120.6 Interest expense 6.5 13.2 — — .4 20.1 Total expenses 546.3 356.9 1.0 18.8 87.0 1,010.0 Pre-tax income (loss) $ 86.5 $ 150.7 $ 9.6 $ (16.6 ) $ (8.2 ) $ 222.0

Investments in Unconsolidated A

Investments in Unconsolidated Affiliates	6 Months Ended
	Jun. 30, 2015
Equity Method Investments and Joint Ventures [Abstract]
Investments in Unconsolidated Affiliates	Investments in Unconsolidated Affiliates White Mountains’s investments in unconsolidated affiliates represent investments in other companies in which White Mountains has a significant voting and economic interest but does not control the entity. Millions June 30, December 31, Symetra common shares $ 395.2 $ 373.8 Unrealized gains from Symetra’s fixed maturity portfolio 2.0 37.6 Carrying value of Symetra common shares 397.2 411.4 Hamer — 3.0 Total investments in unconsolidated affiliates $ 397.2 $ 414.4 Symetra On June 30, 2015, pursuant to the redemption of White Mountains's investments in the Prospector Funds, 513,500 common shares of Symetra were distributed to White Mountains. At June 30, 2015 and December 31, 2014 , White Mountains owned 20,562,379 and 20,048,879 common shares of Symetra Financial Corporation (“Symetra”), a 17.71% and 17.31% common share ownership. White Mountains accounts for its investment in common shares of Symetra using the equity method. During the three and six months ended June 30, 2015 , White Mountains received cash dividends from Symetra of $2.2 million and $4.4 million on its common share investment that were recorded as a reduction of White Mountains’s investment in Symetra. During the three and six months ended June 30, 2014, White Mountains received cash dividends from Symetra of $2.0 million and $4.0 million . As of December 31, 2011, White Mountains concluded that its investment in Symetra common shares was other-than-temporarily impaired and wrote down the GAAP book value of the investment to its estimated fair value of $261.0 million or $15 per share. This impairment, as well as the effect of Symetra capital transactions, has resulted in a basis difference between the GAAP carrying value of White Mountains’s investment in Symetra common shares and the amount derived by multiplying the percentage of White Mountains common share ownership by Symetra’s total GAAP equity. As of June 30, 2015 , the pre-tax unamortized basis difference was $164.1 million , of which $33.0 million is attributable to equity in earnings of unconsolidated affiliates and $131.1 million is attributable to equity in net unrealized gains of unconsolidated affiliates. As of December 31, 2014, the pre-tax unamortized basis difference was $170.4 million , of which $36.4 million is attributable to equity in earnings of unconsolidated affiliates and $134.0 million is attributable to equity in net unrealized gains of unconsolidated affiliates. The pre-tax basis difference is being amortized over a 30 -year period with a weighted average of 28 -years remaining. The amortization is based on estimated future cash flows associated with Symetra’s underlying assets and liabilities to which the basis differences have been attributed. White Mountains continues to record its equity in Symetra's earnings and net unrealized gains (losses). In addition, White Mountains recognizes the amortization of the basis difference through equity in earnings of unconsolidated affiliates and equity in net unrealized gains (losses) from investments in unconsolidated affiliates consistent with the original attribution of the basis differences between equity in earnings and equity in net unrealized gains (losses). For the three and six months ended June 30, 2015 , White Mountains recognized after-tax amortization of $0.7 million and $1.4 million through equity in earnings of unconsolidated affiliates and $2.7 million and $5.5 million through equity in net unrealized gains from investments in unconsolidated affiliates. For the three and six months ended June 30, 2014, White Mountains recognized after-tax amortization of $0.7 million and $1.4 million through equity in earnings of unconsolidated affiliates and $2.9 million and $5.8 million through equity in net unrealized gains from investments in unconsolidated affiliates. The following table summarizes amounts recorded by White Mountains relating to its investment in Symetra for the three and six months ended June 30, 2015 and 2014 : Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Carrying value of investment in Symetra at beginning of period $ 447.3 $ 369.0 $ 411.4 $ 317.3 Equity in earnings (1)(2) 6.1 13.1 13.3 27.5 Equity in net unrealized (losses) gains from Symetra’s fixed maturity portfolio (3) (66.4 ) 35.4 (35.5 ) 74.7 Dividends received (2.2 ) (2.0 ) (4.4 ) (4.0 ) Distribution from Prospector Offshore Fund 12.4 — 12.4 — Carrying value of investment in Symetra at end of period (4)(5) $ 397.2 $ 415.5 $ 397.2 $ 415.5 (1) For the three months ended June 30, 2015 and 2014 , equity in earnings excludes tax expense of $0.4 and $0.9 . For the six months ended June 30, 2015 and 2014 , equity in earnings excludes tax expense of $0.8 and $1.9 . (2) For the three months ended June 30, 2015 and 2014 , equity in earnings includes $0.7 and $0.7 increase relating to the pre-tax amortization of the Symetra common share basis difference. For the six months ended June 30, 2015 and 2014 , equity in earnings includes $1.4 and $1.5 increase relating to the pre-tax amortization of the Symetra common share basis difference. (3) For the three months ended June 30, 2015 and 2014 , net unrealized gains includes $2.8 and $3.1 increase relating to the pre-tax amortization of the Symetra common share basis difference. For the six months ended June 30, 2015 and 2014 , net unrealized gains includes $5.7 and $6.3 increase relating to the pre-tax amortization of the Symetra common share basis difference. (4) Includes White Mountains’s equity in net unrealized gains from Symetra’s fixed maturity portfolio of $2.0 and $31.1 as of June 30, 2015 and 2014 , which excludes tax expense of $0.0 and $2.1 . (5) The aggregate value of White Mountains’s investment in common shares of Symetra was $497.0 based upon the quoted market price of $ 24.17 per share at June 30, 2015 . Hamer On May 27, 2015, White Mountains sold its interest in Hamer LLC, which resulted in a gain of $20.0 million recorded in other revenue. Prior to the sale, White Mountains recorded equity in earnings of $1.1 million and $1.6 million for the three and six months ended June 30, 2015 and $0.4 million and $0.8 million for the three and six months ended June 30, 2014.

Employee Share-Based Incentive

Employee Share-Based Incentive Compensation Plans	6 Months Ended
	Jun. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Employee Share-Based Incentive Compensation Plans	Employee Share-Based Incentive Compensation Plans White Mountains’s Long-Term Incentive Plan (the “WTM Incentive Plan”) provides for grants of various types of share-based and non share-based incentive awards to key employees and service providers of White Mountains. White Mountains’s share-based compensation incentive awards consist of performance shares, restricted shares and stock options. Share-Based Compensation Based on White Mountains Common Shares WTM Performance Shares Performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. Awards generally vest at the end of a three -year period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of common shares at the time awards are paid. The following table summarizes performance share activity for the three and six months ended June 30, 2015 and 2014 for performance shares granted under the WTM Incentive Plan: Three Months Ended June 30, Six Months Ended June 30, 2015 2014 2015 2014 Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 116,396 $ 27.6 127,537 $ 42.3 123,549 $ 48.1 119,220 $ 60.2 Shares paid or expired (1) — — — — (37,977 ) (27.3 ) (37,130 ) (26.7 ) New grants 750 — — — 31,390 — 45,660 — Assumed forfeitures and cancellations (2) (19 ) (.1 ) — (.2 ) 165 .5 (213 ) .2 Expense recognized — 3.6 — 8.4 — 9.8 — 16.8 End of period 117,127 $ 31.1 127,537 $ 50.5 117,127 $ 31.1 127,537 $ 50.5 (1) WTM performance share payments in 2015 for the 2012-2014 performance cycle ranged from 91% to 145.5% of target. WTM performance share payments in 2014 for the 2011-2013 performance cycle ranged from 88% to 131.5% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP. For the 2012-2014 performance cycle, all performance shares earned were settled in cash. For the 2011-2013 performance cycle, the Company issued common shares for 3,570 performance shares earned and all other performance shares earned were settled in cash If the outstanding WTM performance shares had vested on June 30, 2015 , the total additional compensation cost to be recognized would have been $25.9 million , based on accrual factors at June 30, 2015 (common share price and payout assumptions). Performance Shares granted under the WTM Incentive Plan The following table summarizes performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan at June 30, 2015 for each performance cycle: Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Performance cycle: 2015 – 2017 31,390 $ 3.4 2014 – 2016 41,937 5.1 2013 – 2015 46,803 23.4 Sub-total 120,130 31.9 Assumed forfeitures (3,003 ) (.8 ) Total at June 30, 2015 117,127 $ 31.1 Restricted Shares The following table summarizes the unrecognized compensation cost associated with the outstanding restricted share awards for the three and six months ended June 30, 2015 and 2014 : Three Months Ended June 30, Six Months Ended June 30, 2015 2014 2015 2014 Millions, except share amounts Restricted Unamortized Restricted Unamortized Restricted Shares Unamortized Issue Date Fair Value Restricted Shares Unamortized Issue Date Fair Value Non-vested, Beginning of period 71,125 $ 26.5 81,325 $ 24.8 83,314 $ 14.3 94,130 $ 17.0 Issued 750 .5 — — 23,640 15.7 20,400 11.8 Vested — — — — (35,079 ) — (33,205 ) — Forfeited — — — — — — — — Expense recognized — (3.8 ) — (3.9 ) — (6.8 ) — (7.9 ) End of period 71,875 $ 23.2 81,325 $ 20.9 71,875 $ 23.2 81,325 $ 20.9 During the first six months of 2015, White Mountains issued 23,640 restricted shares that vest on January 1, 2018. During the first six months of 2014, White Mountains issued 20,400 restricted shares that vest on January 1, 2017. The unrecognized compensation cost at June 30, 2015 is expected to be recognized ratably over the remaining vesting periods. Share-Based Compensation Based on OneBeacon Ltd. Common Shares The OneBeacon Long-Term Incentive Plan (the “OneBeacon Incentive Plan”) provides for grants to key employees of OneBeacon various types of share-based and non share-based incentive awards. OneBeacon’s share-based incentive awards include OneBeacon performance shares and restricted shares. OneBeacon Performance Shares OneBeacon performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. OneBeacon performance share awards generally vest at the end of a three -year period, are subject to the attainment of pre-specified performance goals, and are valued based on the market value of OneBeacon Ltd. common shares at the time awards are paid. The following table summarizes performance share activity for the three and six months ended June 30, 2015 and 2014 for OneBeacon performance shares granted under the OneBeacon Incentive Plan: Three Months Ended June 30, Six Months Ended June 30, 2015 2014 2015 2014 Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 462,147 $ 2.1 512,938 $ 3.6 517,470 $ 3.4 493,421 $ 4.0 Shares paid or expired (1) — — — — (181,290 ) (1.5 ) (142,138 ) (1.0 ) New grants — — — — 154,887 — 165,800 — Assumed forfeitures and cancellations (2) — — — — (28,920 ) — (4,145 ) — Expense recognized — .3 — .8 — .5 — 1.4 End of period 462,147 $ 2.4 512,938 $ 4.4 462,147 $ 2.4 512,938 $ 4.4 (1) OneBeacon performance share payments in 2015 for the 2012-2014 performance cycle were at 45.7% of target. OneBeacon performance share payments in 2014 for the 2011-2013 performance cycle were at 37.1% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP. If the outstanding OneBeacon performance shares had been vested on June 30, 2015 , the total additional compensation cost to be recognized would have been $2.8 million , based on accrual factors at June 30, 2015 (common share price, accumulated dividends and payout assumptions). The following table summarizes OneBeacon performance shares outstanding awarded under the OneBeacon Incentive Plan at June 30, 2015 for each performance cycle: Millions, except share amounts Target Accrued Expense Performance cycle: 2015 – 2017 154,887 $ .3 2014 – 2016 151,810 .6 2013 – 2015 167,300 1.5 Sub-total 473,997 2.4 Assumed forfeitures (11,850 ) — Total at June 30, 2015 462,147 $ 2.4 OneBeacon Restricted Shares The following table summarizes the unrecognized compensation cost associated with the outstanding OneBeacon restricted stock awards for the three and six months ended June 30, 2015 and 2014 : Three Months Ended June 30, Six Months Ended June 30, 2015 2014 2015 2014 Millions, except share amounts Restricted Unamortized Restricted Unamortized Restricted Unamortized Restricted Unamortized Non-vested, Beginning of period 390,950 $ 4.0 612,500 $ 5.7 612,500 $ 3.5 915,000 $ 6.5 Issued — — — — 75,950 1.1 — — Vested — — — — (296,000 ) — (300,000 ) — Forfeited — — — — (1,500 ) — (2,500 ) — Expense recognized — (.5 ) — (.8 ) — (1.1 ) — (1.6 ) End of period 390,950 $ 3.5 612,500 $ 4.9 390,950 $ 3.5 612,500 $ 4.9 On February 24, 2015, OneBeacon issued 75,950 restricted shares that fully vest on January 1, 2018. On March 1, 2012, OneBeacon issued 300,000 restricted shares that vest in two equal annual installments. The first installment vested on February 28, 2014. On May 25, 2011, OneBeacon issued 630,000 restricted shares to its CEO that vest in four equal annual installments. The first installment vested on February 22, 2014. Concurrently with the grant of the restricted shares, 35,000 OneBeacon performance shares issued to OneBeacon’s CEO for the 2011-2013 performance share cycle were forfeited and performance share awards to OneBeacon’s CEO for the subsequent five years have been or will also be reduced by 35,000 shares. The unrecognized compensation cost at June 30, 2015 is expected to be recognized ratably over the remaining vesting periods. OneBeacon Restricted Stock Units During the first six months of 2015, the OneBeacon Compensation Committee awarded to certain employees 214,430 restricted stock units (“RSUs”), of which, net of forfeiture assumptions, 209,369 were outstanding as of June 30, 2015. The RSUs are scheduled to vest on January 1, 2018 and will be paid out in cash or shares at the discretion of the OneBeacon Compensation Committee. For the three and six months ended June 30, 2015 , the expense associated with the RSUs was $0.3 million and $0.4 million .

Fair Value of Financial Instrum

Fair Value of Financial Instruments	6 Months Ended
	Jun. 30, 2015
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	Fair Value of Financial Instruments White Mountains accounts for its financial instruments at fair value with the exception of the OBH Senior Notes and the SIG Senior Notes, which are recorded as debt at face value less unamortized original issue discount, and the SIG Preference Shares, which are recorded as non-controlling interest at face value. The following table summarizes the fair value and carrying value of financial instruments as of June 30, 2015 and December 31, 2014 : June 30, 2015 December 31, 2014 Millions Fair Value Carrying Value Fair Value Carrying Value 2012 OBH Senior Notes $ 280.4 $ 274.7 $ 286.0 $ 274.7 SIG Senior Notes 437.8 399.7 437.8 399.7 SIG Preference Shares 261.8 250.0 260.0 250.0 Tranzact Bank Facility 70.2 70.6 68.7 68.7 The fair value estimate for the 2012 OBH Senior Notes has been determined using quoted market prices. The 2012 OBH Senior Notes are considered a Level 2 measurement based upon the volume and frequency of observable transactions. The fair value estimates for the SIG Senior Notes and the SIG Preference Shares have been determined based on indicative broker quotes and are considered to be Level 3 measurements. The fair value estimate for the Tranzact Bank Facility has been determined based on a discounted cash flows approach and is considered to be a Level 3 measurement.

Discontinued Operations

Discontinued Operations	6 Months Ended
	Jun. 30, 2015
Discontinued Operations and Disposal Groups [Abstract]
Discontinued Operations	Discontinued Operations For the three months ended June 30, 2015 , White Mountains recorded a net loss from sale of discontinued operations of $0.1 million , which was primarily related to a tax payment of $0.3 million to Allstate pursuant to the Esurance sale and post-closing expenses incurred in connection with the Runoff Business, offset by a $0.3 million gain related to an adjustment to the estimated loss on sale from the Runoff Transaction, which includes the final settlement of certain post-closing items. The six months ended June 30, 2015 also included a net gain from sale of discontinued operations of $8.0 million , which is primarily related to the favorable loss reserve development on loss reserves transferred in the sale of Esurance and Answer Financial. See “ Esurance ” in Note 18. For both the three and six months ended June 30, 2014, White Mountains recorded a net gain from sale of discontinued operations of $2.7 million , primarily due to an interim payment on the favorable development on loss reserves transferred with the sale of Esurance and Answer Financial. The gain from sale of discontinued operations for both the three and six months ended June 30, 2014 also included two offsetting amounts at OneBeacon. First, OneBeacon reduced its estimate of the fair value of the surplus notes expected to be issued at the closing of the Runoff Transaction based on their internal valuation model which, as of June 30, 2014, was $8.2 million , pre-tax, ( $5.3 million , after tax) less than par value of the surplus notes, resulting in a corresponding increase to the estimated loss on sale. Second, the stock purchase agreement was revised during the second quarter of 2014 in part to increase the cap on seller financing, which resulted in OneBeacon recording a $7.4 million reserve charge ( $4.8 million after tax) as a reduction to the estimated loss on sale of discontinued operations. Earnings Per Share Basic earnings per share amounts are based on the weighted average number of common shares outstanding including unvested restricted shares that are considered participating securities. Diluted earnings per share amounts are based on the weighted average number of common shares including unvested restricted shares and the net effect of potentially dilutive common shares outstanding. The following table outlines the computation of earnings per share for discontinued operations for the three and six months ended June 30, 2015 and 2014 : Three Months Ended Six Months Ended June 30, June 30, 2015 2014 2015 2014 Basic and diluted earnings per share numerators (in millions): Net income attributable to White Mountains’s common shareholders $ (.3 ) $ 2.6 $ 7.6 $ 2.1 Allocation of income for participating unvested restricted common shares (1) — — (.1 ) — Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts (2) $ (.3 ) $ 2.6 $ 7.5 $ 2.1 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 5,985.7 6,162.2 5,982.1 6,166.0 Average unvested restricted common shares (3) (71.4 ) (81.3 ) (65.2 ) (75.9 ) Basic earnings per share denominator 5,914.3 6,080.9 5,916.9 6,090.1 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 5,985.7 6,162.2 5,982.1 6,166.0 Average unvested restricted common shares (3) (71.4 ) (81.3 ) (65.2 ) (75.9 ) Average outstanding dilutive options to acquire common shares (4) — — — — Diluted earnings per share denominator 5,914.3 6,080.9 5,916.9 6,090.1 Basic and diluted earnings per share (in dollars): $ (.04 ) $ .43 $ 1.27 $ .34 (1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (2) Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three and six months ended June 30, 2015 and 2014 . (3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date. (See Note 15 - “Employee Share-Based Compensation Plans” ). (4) The diluted earnings per share denominator for the three and six months ended June 30, 2015 and 2014 does not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.

Contingencies

Contingencies	6 Months Ended
	Jun. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Contingencies Legal Contingencies White Mountains, and the insurance and reinsurance industry in general, are routinely subject to claims related litigation and arbitration in the normal course of business, as well as litigation and arbitration that do not arise from, or are directly related to, claims activity. White Mountains’s estimates of the costs of settling matters routinely encountered in claims activity are reflected in the reserves for unpaid loss and LAE. (See Note 3 - “Loss and Loss Adjustment Expense Reserves”) . White Mountains considers the requirements of ASC 450 when evaluating its exposure to non-claims related litigation and arbitration. ASC 450 requires that accruals be established for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. ASC 450 also requires that litigation and arbitration be disclosed if it is probable that a loss has been incurred or if there is a reasonable possibility that a loss may have been incurred. Although the ultimate outcome of claims and non-claims related litigation and arbitration, and the amount or range of potential loss at any particular time, is often inherently uncertain, management does not believe that the ultimate outcome of such claims and non-claims related litigation and arbitration will have a material adverse effect on White Mountains’s financial condition, results of operations or cash flows. The following summarizes significant legal contingencies, ongoing non-claims related litigation or arbitration as of June 30, 2015 : Esurance On October 7, 2011, the Company completed the sale of its Esurance and Answer Financial subsidiaries (the “Transferred Subsidiaries”) to The Allstate Corporation (“Allstate”) pursuant to a Stock Purchase Agreement dated as of May 17, 2011 (filed as an exhibit to the Company’s current report on Form 8-K on May 18, 2011, the “Agreement”). Subject to specified thresholds and limits, the Company remains contingently liable to Allstate for specified matters related to the pre-closing period, including (a) specified litigation matters, (b) losses of the Transferred Subsidiaries arising from extra-contractual claims and claims in excess of policy limits (“ECO/EPL losses”), (c) certain corporate reorganizations effected to remove entities from the Transferred Subsidiaries that were not being sold in the transaction, and (d) certain tax matters, including certain net operating losses being less than stated levels. In addition, the Company retains 90% of positive or negative development in the loss reserves of the Transferred Subsidiaries as of the closing date (net of ECO/EPL losses) through December 31, 2014 (the “Reserve Settlement”). The transferred loss reserves have developed favorably since the closing and, as a result, Allstate has made payments to the Company in the two annual interim settlements since closing. For the third (and final) reserve settlement, Allstate believes it owes approximately $8.0 million to the Company for favorable development in the transferred loss reserves, which the Company recorded as a gain from sale of discontinued operations in the first quarter of 2015. The Company believes that it is owed approximately $18.0 million for the final settlement, and the parties have commenced the process of arbitrating the dispute. Runoff Transaction Subsequent to the closing of the Runoff Transaction, on January 22, 2015, three holders of insurance policies issued by the companies OneBeacon sold to Armour filed a Petition for Review with the Commonwealth Court of Pennsylvania (“Commonwealth Court”) requesting that the Commonwealth Court vacate the Pennsylvania Insurance Department’s (“PID”) orders approving the Runoff Transaction and denying their right to intervene in the PID’s regulatory review of the Runoff Transaction. White Mountains believes the claims made by the petitioners are without merit and has intervened in the proceedings before the Commonwealth Court to vigorously defend the propriety of the PID’s orders in their entirety. White Mountains believes that the possibility is remote that these proceedings could result in an adverse outcome or have a material financial impact on the Company’s results of operations or financial position in the future. Tribune Company In June 2011, Deutsche Bank Trust Company Americas, Law Debenture Company of New York and Wilmington Trust Company (collectively referred to as “Plaintiffs”), in their capacity as trustees for certain senior notes issued by the Tribune Company (“Tribune”), filed lawsuits in various jurisdictions (the “Noteholder Actions”) against numerous defendants including OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune seeking recovery of the proceeds from the sale of common stock of Tribune in connection with Tribune's leveraged buyout in 2007 (the “LBO”). Tribune filed for bankruptcy in 2008 in the Delaware bankruptcy court (the “Bankruptcy Court”). The Bankruptcy Court granted Plaintiffs permission to commence these LBO-related actions, and in 2011, the Judicial Panel on Multidistrict Litigation granted a motion to consolidate the actions for pretrial matters and transferred all such proceedings to the United States District Court for the Southern District of New York. Plaintiffs seek recovery of the proceeds received by the former Tribune shareholders on a theory of constructive fraudulent transfer asserting that Tribune purchased or repurchased its common shares without receiving fair consideration at a time when it was, or as a result of the purchases of shares, was rendered, insolvent. OneBeacon has entered into a joint defense agreement with other affiliates of White Mountains that are defendants in the action. Certain subsidiaries of White Mountains received approximately $39.0 million for Tribune common stock tendered in connection with the LBO. The Court granted an omnibus motion to dismiss the Noteholder Actions in September 2013 and Plaintiffs appeal is pending. In addition, OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune, along with thousands of former Tribune shareholders, have been named as defendants in an adversary proceeding brought by the Official Committee of Unsecured Creditors of the Tribune Company (the “Committee”), on behalf of the Tribune Company, which seeks to avoid the repurchase of shares by Tribune in the LBO on a theory of intentional fraudulent transfer (the “Committee Action”). Tribune emerged from bankruptcy in 2012, and a litigation trustee replaced the Committee as plaintiff in the Committee Action. This matter was consolidated for pretrial matters with the Noteholder Actions in the United States District Court for the Southern District of New York and was stayed pending the motion to dismiss in the Noteholder Action. An omnibus motion to dismiss the shareholder defendants in the Committee Action was filed in May 2014. No amount has been accrued in connection with this matter as of June 30, 2015, as the amount of loss, if any, cannot be reasonably estimated.

Subsequent Events (Notes)

Subsequent Events (Notes)	6 Months Ended
	Jun. 30, 2015
Subsequent Events [Abstract]
Subsequent Events [Text Block]	Note 19. Subsequent Events Sale of Sirius Group On July 27, 2015, White Mountains announced it signed a definitive agreement to sell Sirius Group to CM International Holding PTE Ltd., the Singapore-based investment arm of China Minsheng Investment Corp., Ltd. (“CMI”). The purchase price will be paid in cash in an amount equal to 127.3% of Sirius’s closing date tangible common shareholder’s equity plus $10.0 million . Based on Sirius's tangible common shareholder's equity at December 31, 2014, the purchase price would be $2.2 billion . White Mountains has the option to replenish Sirius’s tangible common shareholder’s equity to its December 31, 2014 level should it be below that level at closing. The transaction is expected to close within six months. It is subject to regulatory approval and other customary closing conditions. In connection with the transaction, White Mountains caused Sirius Group to purchase several industry loss warranty (“ILW”) contracts to augment its overall retrocessional program in order to reduce the risk of loss from major catastrophes subsequent to the signing of the definitive agreement to sell Sirius Group. The cost and potential economic benefit provided by the coverage under the ILWs inures to White Mountains. The majority of the contracts expire in May or June of 2016. The following summarizes the ILW contracts in force: Scope Limit Industry Loss Trigger United States first event $75.0 million $40.0 billion United States first event $22.5 million $50.0 billion United States second event $45.0 million $15.0 billion Japan first event $25.0 million $12.5 billion OneBeacon Crop Business On July 31, 2015, OneBeacon exited its multiple peril crop insurance (“MPCI”) and its related crop-hail businesses (collectively, “Crop Business”), due to the sale of its exclusive partner, Climate Crop Insurance Agency, to an affiliate of AmTrust Financial Services (“AmTrust”). As a result of the sale, OneBeacon and Climate Crop Insurance Agency agreed to terminate the existing 5 year agreement early and OneBeacon received a payment of $3.0 million in consideration. Also related to the sale, OneBeacon has withdrawn its 2016 Plan of Operations from the Federal Crop Insurance Corporation, which previously authorized it to write MPCI for the 2016 Reinsurance Year (July 1, 2015 to June 30, 2016) and affiliates of AmTrust will reinsure OneBeacon's remaining net Crop Business exposure for the 2015 reinsurance year (July 1, 2014 to June 30, 2015) under related 100% quota share reinsurance agreements. During the six months ended June 30, 2015, OneBeacon recorded net written premiums related to the Crop business of $ 26.6 million and a net pre-tax underwriting loss $1.3 million (excluding allocated other underwriting expenses of $1.5 million ). During the year ended December 31, 2014, OneBeacon recorded net written premiums related to the Crop business of $35.1 million and a net pre-tax underwriting loss of $3.5 million (excluding allocated other underwriting expenses of $1.0 million ).

Summary of Significant Accoun27

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2015
Accounting Policies [Abstract]
Recently Adopted Changes in Accounting Principles and Recently Issued Accounting Pronouncements	Recently Adopted Changes in Accounting Principles Qualified Affordable Housing Projects Effective January 1, 2015, White Mountains adopted ASU 2014-01, Accounting for Investments in Qualified Affordable Housing Projects (“QAHP”) (ASC 323). ASU 2014-01 allows investors in QAHP to make a policy election to use the proportional amortization method. Under the proportional amortization method, the investor amortizes the initial cost of the investment in proportion to the tax credits and other tax benefits received and recognizes the investment results, net of the related tax benefits, as a component of income tax expense. Prior to adoption, White Mountains accounted for its QAHP investment under the equity method and recognized its share of its QAHP investment's losses in investment income. White Mountains made the policy election to account for its investment in its QAHP investment using the proportional amortization method, applied retrospectively. Under the proportional amortization method, the cumulative loss recognized through December 31, 2014 and December 31, 2013 was $0.9 million and $0.5 million . The retrospective adoption resulted in an increase of $0.5 million and $0.9 million to net investment income and a net increase of $0.6 million and $1.2 million to income tax expense for the three and six months ended June 30, 2014. Footnote disclosures for prior year amounts have been amended to be consistent with the restated amounts described above. Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity On April 10, 2014, the FASB issued ASU 2014-08, Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity (ASC 205 and ASC 360) to reduce diversity in practice for reporting discontinued operations. ASU 2014-08 limits discontinued operations treatment to disposals that represent a strategic shift and that have a major effect on a reporting entity’s operations and financial results to be reported as discontinued operations. The revised guidance also requires expanded disclosure in the financial statements for discontinued operations as well as for disposals of significant components of an entity that do not qualify for discontinued operations presentation. White Mountains has not had any transactions that occurred since ASU 2014-08 became effective on December 15, 2014. Pushdown Accounting ASU 2014-17, Pushdown Accounting, a consensus of the FASB Emerging Issues Task Force (ASC 805) became effective upon its issuance on November 18, 2014 . The new guidance, which is applicable prospectively, gives an acquired non-public company the option to apply pushdown accounting in its separate company financial statements in the period in which it is acquired in a change of control transaction. Once pushdown accounting has been applied, the election is irreversible. Acquired entities that chose not to apply pushdown accounting at the time of acquisition, may apply pushdown accounting in a subsequent period as a change in accounting principle under ASC 250, Accounting Changes and Error Corrections . White Mountains has not had any acquisitions since AUS 2014-17 became effective. Unrecognized Tax Benefits Effective January 1, 2014, White Mountains adopted ASU 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists (ASC 740). The new ASU requires balance sheet presentation of an unrecognized tax benefit as a reduction of a deferred tax asset for a net operating loss (“NOL”) carryforward or tax credit carryforward rather than as a liability. The exception is in circumstances where a carryforward is not available to settle the additional taxes that might arise upon disallowance of the tax position under the tax law of the applicable jurisdiction. Prior to the issuance of ASU 2013-11, the guidance for unrecognized tax benefits under ASC 740 did not provide explicit guidance on whether an entity should present an unrecognized tax benefit as a liability or as a reduction of NOL carryforwards or other tax credits. In circumstances where an NOL carryforward is not available to offset settlement of any additional taxes arising from a disallowed tax position, the unrecognized tax benefit should be presented as a liability. The new guidance became effective for White Mountains on January 1, 2014. Adoption did not have any impact on White Mountains's financial condition, results of operations or cash flows or financial statement presentation. Recently Issued Accounting Pronouncements Short-Duration Contracts On May 21, 2015, the FASB issued ASU 2015-09, Disclosures about Short Duration Contracts (ASC 944) which requires expanded footnote disclosures about loss and loss adjustment expense (“LAE”) reserves. Under the new guidance, some disclosures currently presented outside of White Mountains’s financial statements, such as loss development tables and a reconciliation of loss development data to the loss and LAE reserves reflected on the balance sheet, will become part of the financial statement footnotes. In addition, the loss development tables required to be presented under the new ASU must be presented on a disaggregated basis by accident year rather than by reporting year as currently presented. Some of the expanded disclosures are new requirements, such as the disclosure of reserves for losses incurred but not reported (“IBNR”) plus expected development on reported claims, which must be presented by accident year on a disaggregated basis. The new guidance also requires new disclosures about claim frequency data together with descriptions of the approach used to measure that data. Qualitative descriptions of methodologies and assumptions used to develop IBNR estimates must be presented together with the disaggregated amounts of IBNR to which they relate, along with a discussion of any significant changes in methodology and assumptions and the related effect upon the loss reserves. The new guidance will be effective for annual periods beginning after December 15, 2015 and interim periods within annual periods beginning after December 15, 2016 with retrospective restatement of prior periods required. White Mountains will modify its financial statement footnote disclosures to conform to the requirements of ASU 2015-09 upon adoption, including revisions to prior year’s disclosures. Fair Value Measurements On May 1, 2015, the FASB issued ASU 2015-07, Fair Value Measurement - Disclosures for Investments in Certain Entities that Calculate Net Asset Value per Share (or Its Equivalent) (ASC 820) which eliminates the requirement to disclose the fair value hierarchy level for investments for which fair value is measured at net asset value using the practical expedient in ASC 820. White Mountains measures the fair value of its investments in hedge funds and private equity funds using this practical expedient and has classified those measurements within Level 3 of the fair value hierarchy. The new guidance is effective for fiscal years beginning after December 15, 2015 and interim periods within those fiscal years. Debt Issuance Costs On April 7, 2015, the FASB issued ASU 2015-03, Imputation of Interest (ASC 835) which requires debt issuance costs related to a recognized debt liability to be presented as a deduction from the carrying amount of the related debt, consistent with the treatment required for debt discounts. ASU 2015-03 is effective for annual and interim reporting periods beginning after December 15, 2015. White Mountains does not expect ASU 2015-03 to impact its financial position, results of operations, cash flows, presentation and disclosures. Amendments to Consolidation Analysis On February 18, 2015, the FASB issued ASU 2015-02, Amendments to the Consolidation Analysis (ASC 810) which amends the guidance for determining whether an entity is a variable interest entity (“VIE”). ASU 2015-02 eliminates the separate consolidation guidance for limited partnerships and with it, the presumption that a general partner should consolidate a limited partnership. In addition, ASU 2015-02 changes the guidance for determining if fee arrangements qualify as variable interests and the effect fee arrangements have on the determination of the primary beneficiary. ASU 2015-02 is effective for annual and interim reporting periods beginning after December 15, 2015 and must be applied retrospectively. White Mountains does not expect ASU 2015-02 to affect the consolidation analysis for any of its existing investments. Share-Based Compensation Awards On June 19, 2014, the FASB issued ASU 2014-12, Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period (ASC 718) . The new guidance is intended to eliminate diversity in practice for employee share-based awards containing performance targets that could be achieved after the requisite service period. Some reporting entities account for performance targets that can be achieved after the requisite service period as performance conditions that affect the vesting of the award while other reporting entities treat those performance targets as non-vesting conditions that affect the grant-date fair value of the award. The updated guidance requires that a performance target that affects vesting and that can be achieved after the requisite service period be treated as a performance condition. Compensation cost should be recognized in the period it becomes probable that the performance target will be achieved and should represent the compensation cost attributable to the periods for which service has been rendered. The new guidance is effective for annual and interim reporting periods beginning after December 15, 2015. White Mountains does not expect adoption to have a significant effect on its financial position, results of operations, cash flows, presentation or disclosures. Revenue Recognition On May 28, 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (ASC 606) , which modifies the guidance for revenue recognition. The scope of the new ASU excludes insurance contracts but is applicable to certain fee arrangements, such as third-party investment management fees charged by White Mountains Advisors, which were $2.0 million and $4.0 million for the three and six months ended June 30, 2015 and $3.0 million and $6.0 million for the three and six months ended June 30, 2014. White Mountains is in the process of evaluating the new guidance and has not yet determined the potential effect of adoption on its financial position, results of operations, or cash flows. ASU 2014-09 is effective for annual and interim reporting periods beginning after December 15, 2016.

Loss and Loss Adjustment Expe28

Loss and Loss Adjustment Expense Reserves (Tables)	6 Months Ended
	Jun. 30, 2015
Insurance Loss Reserves [Abstract]
Loss and loss adjustment expense ("LAE") reserve activities	The following table summarizes the loss and loss adjustment expense (“LAE”) reserve activities of White Mountains’s insurance and reinsurance subsidiaries for the three and six months ended June 30, 2015 and 2014 : Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Gross beginning balance $ 3,035.0 $ 3,048.4 $ 3,159.8 $ 3,079.3 Less beginning reinsurance recoverable on unpaid losses (436.9 ) (425.2 ) (483.9 ) (428.1 ) Net loss and LAE reserves 2,598.1 2,623.2 2,675.9 2,651.2 Loss and LAE reserves consolidated — SSIE — — — 13.6 Loss and LAE incurred relating to: Current year losses 300.0 278.4 566.8 518.9 Prior year losses (11.7 ) 4.3 (13.1 ) (6.9 ) Total incurred losses and LAE 288.3 282.7 553.7 512.0 Foreign currency translation adjustment to loss and LAE reserves 7.5 (.5 ) (14.9 ) 1.3 Loss and LAE paid relating to: Current year losses (72.9 ) (65.8 ) (113.5 ) (100.8 ) Prior year losses (216.7 ) (219.5 ) (496.9 ) (457.2 ) Total loss and LAE payments (289.6 ) (285.3 ) (610.4 ) (558.0 ) Net ending balance 2,604.3 2,620.1 2,604.3 2,620.1 Plus ending reinsurance recoverable on unpaid losses 427.5 433.2 427.5 433.2 Gross ending balance $ 3,031.8 $ 3,053.3 $ 3,031.8 $ 3,053.3

Investment Securities (Tables)

Investment Securities (Tables)	6 Months Ended
	Jun. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Pre-tax net investment income	Pre-tax net investment income for the three and six months ended June 30, 2015 and 2014 consisted of the following: Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Investment income: Fixed maturity investments $ 23.4 $ 25.6 $ 46.5 $ 48.0 Short-term investments .7 .6 1.3 1.2 Common equity securities 3.3 5.5 6.7 10.9 Convertible fixed maturity and preferred investments .1 .8 .3 1.2 Other long-term investments .9 1.9 .8 2.6 Interest on funds held under reinsurance treaties — — (.1 ) — Total investment income 28.4 34.4 55.5 63.9 Third-party investment expenses (4.6 ) (4.8 ) (8.4 ) (9.6 ) Net investment income, pre-tax $ 23.8 $ 29.6 $ 47.1 $ 54.3
Schedule of Realized and Unrealized Gain (Loss) on Investments	Net realized and unrealized investment gains and losses for the three and six months ended June 30, 2015 and 2014 consisted of the following: Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Net realized investment gains, pre-tax $ 112.9 $ 30.5 $ 165.7 $ 52.2 Net unrealized investment (losses) gains, pre-tax (174.0 ) 83.4 (123.4 ) 125.5 Net realized and unrealized investment (losses) gains, pre-tax (61.1 ) 113.9 42.3 177.7 Income tax benefit (expense) attributable to net realized and unrealized investment gains (losses) 16.5 (23.0 ) (9.8 ) (38.7 ) Net realized and unrealized investment (losses) gains, after tax $ (44.6 ) $ 90.9 $ 32.5 $ 139.0
Realized Gain (Loss) on Investments	Net realized investment gains (losses) for the three and six months ended June 30, 2015 and 2014 consisted of the following: Three Months Ended Three Months Ended June 30, 2015 June 30, 2014 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ 2.7 $ 14.3 $ 17.0 $ 5.7 $ 3.0 $ 8.7 Short-term investments — 5.7 5.7 — — — Common equity securities 65.9 7.2 73.1 19.4 — 19.4 Convertible fixed maturity and preferred investments .6 — .6 1.5 — 1.5 Other long-term investments 16.9 — 16.9 .6 .3 .9 Forward contracts (.4 ) — (.4 ) — — — Net realized investment gains, pre-tax 85.7 27.2 112.9 27.2 3.3 30.5 Income tax (expense) attributable to net realized investment gains (24.6 ) (6.4 ) (31.0 ) (5.3 ) (1.0 ) (6.3 ) Net realized investment gains, after tax $ 61.1 $ 20.8 $ 81.9 $ 21.9 $ 2.3 $ 24.2 Six Months Ended Six Months Ended June 30, 2015 June 30, 2014 Millions Net Net Total net realized Net Net Total net realized Fixed maturity investments $ 12.4 $ 35.5 $ 47.9 $ 10.1 $ (.5 ) $ 9.6 Short-term investments — 9.3 9.3 — — — Common equity securities 90.3 8.3 98.6 38.3 (.1 ) 38.2 Convertible fixed maturity and preferred investments (3.6 ) — (3.6 ) 3.9 — 3.9 Other long-term investments 12.8 1.1 13.9 .3 .3 .6 Forward contracts (.4 ) — (.4 ) (.1 ) — (.1 ) Net realized investment gains (losses), pre-tax 111.5 54.2 165.7 52.5 (.3 ) 52.2 Income tax (expense) attributable to net realized investment gains (losses) (31.1 ) (13.1 ) (44.2 ) (9.9 ) (.1 ) (10.0 ) Net realized investment gains (losses), after tax $ 80.4 $ 41.1 $ 121.5 $ 42.6 $ (.4 ) $ 42.2
Schedule of unrealized gain (loss) on investments	The following table summarizes net unrealized investment gains (losses) for the three and six months ended June 30, 2015 and 2014 : Three Months Ended Three Months Ended June 30, 2015 June 30, 2014 Millions Net unrealized (losses) Net Total net unrealized (losses) reflected in earnings Net unrealized gains (losses) Net Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ (34.7 ) $ (39.0 ) $ (73.7 ) $ 24.9 $ 28.2 $ 53.1 Short-term investments — — — — — — Common equity securities (63.9 ) (8.2 ) (72.1 ) 21.0 1.4 22.4 Convertible fixed maturity and preferred investments — (.1 ) (.1 ) (2.4 ) .3 (2.1 ) Other long-term investments (29.1 ) 1.0 (28.1 ) 8.7 1.3 10.0 Forward contracts — — — — — — Net unrealized investment (losses) (127.7 ) (46.3 ) (174.0 ) 52.2 31.2 83.4 Income tax benefit (expense) 36.0 11.5 47.5 (9.8 ) (6.9 ) (16.7 ) Net unrealized investment $ (91.7 ) $ (34.8 ) $ (126.5 ) $ 42.4 $ 24.3 $ 66.7 Six Months Ended Six Months Ended June 30, 2015 June 30, 2014 Millions Net unrealized (losses) gains Net Total net unrealized (losses) gains reflected in earnings Net unrealized gains (losses) Net Total net unrealized gains (losses) reflected in earnings Fixed maturity investments $ (22.6 ) $ 1.2 $ (21.4 ) $ 44.3 $ 40.4 $ 84.7 Short-term investments — — — — — — Common equity securities (68.1 ) (7.5 ) (75.6 ) 25.0 1.9 26.9 Convertible fixed maturity and preferred investments .7 — .7 (1.7 ) .3 (1.4 ) Other long-term investments (28.2 ) 1.1 (27.1 ) 13.6 1.7 15.3 Forward contracts — — — — — — Net unrealized investment (losses) gains, pre-tax (118.2 ) (5.2 ) (123.4 ) 81.2 44.3 125.5 Income tax benefit (expense) attributable to net unrealized investment (losses) gains 33.2 1.2 34.4 (18.9 ) (9.8 ) (28.7 ) Net unrealized investment (losses) gains, after tax $ (85.0 ) $ (4.0 ) $ (89.0 ) $ 62.3 $ 34.5 $ 96.8
Net unrealized investment gains (losses) for Level 3 investments	The following table summarizes the amount of total pre-tax unrealized investment (losses) gains included in earnings for Level 3 investments for the three and six months ended June 30, 2015 and 2014 : Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Fixed maturity investments $ .3 $ .2 $ (.4 ) $ .4 Common equity securities 4.9 2.0 3.1 2.8 Convertible fixed maturity and preferred investments .7 3.2 .8 3.2 Other long-term investments (27.2 ) 8.9 (30.0 ) 15.0 Total unrealized investment (losses) gains, pre-tax - Level 3 investments $ (21.3 ) $ 14.3 $ (26.5 ) $ 21.4
Investment holdings, equity securities, convertible fixed maturities and other long-term investments	The cost or amortized cost, gross unrealized investment gains and losses, net foreign currency gains and losses, and carrying values of White Mountains’s fixed maturity investments as of June 30, 2015 and December 31, 2014 were as follows: June 30, 2015 Millions Cost or amortized cost (1) Gross unrealized gains Gross unrealized losses Net foreign currency (losses) gain (1) Carrying value U.S. Government and agency obligations $ 183.9 $ .3 $ (.2 ) $ (.4 ) $ 183.6 Debt securities issued by corporations 2,232.7 27.7 (7.0 ) 57.7 2,311.1 Municipal obligations 118.0 .8 (1.1 ) — 117.7 Mortgage-backed and asset-backed securities 2,057.4 9.2 (4.7 ) 15.5 2,077.4 Foreign government, agency and provincial obligations 98.9 .3 (1.1 ) .6 98.7 Preferred stocks 79.5 6.3 — .2 86.0 Total fixed maturity investments $ 4,770.4 $ 44.6 $ (14.1 ) $ 73.6 $ 4,874.5 (1) The cost of investment securities is determined using the functional currency of the White Mountains consolidated entity that holds the investment. As a result, the table includes unrealized foreign currency gains from U.S. dollar denominated investments held at Sirius Group. December 31, 2014 Millions Cost or amortized cost (1) Gross unrealized gains Gross unrealized losses Net foreign currency gains (losses) (1) Carrying value U.S. Government and agency obligations $ 184.7 $ .1 $ (.3 ) $ 3.6 $ 188.1 Debt securities issued by corporations 2,221.3 45.2 (5.1 ) 49.8 2,311.2 Municipal obligations 82.0 1.4 (.2 ) — 83.2 Mortgage-backed and asset-backed securities 1,811.1 7.6 (3.5 ) 25.7 1,840.9 Foreign government, agency and provincial obligations 274.6 4.2 (1.0 ) (2.7 ) 275.1 Preferred stocks 79.6 6.1 — .1 85.8 Total fixed maturity investments $ 4,653.3 $ 64.6 $ (10.1 ) $ 76.5 $ 4,784.3 (1) The cost of investment securities is determined using the functional currency of the White Mountains consolidated entity that holds the investment. As a result, the table includes unrealized foreign currency gains from U.S. dollar denominated investments held at Sirius Group. The cost or amortized cost, gross unrealized investment gains and losses, net foreign currency gains and losses, and carrying values of White Mountains’s common equity securities, convertible fixed maturity and preferred investments and other long-term investments as of June 30, 2015 and December 31, 2014 were as follows: June 30, 2015 Millions Cost or amortized cost Gross unrealized gains Gross unrealized losses Net foreign Carrying value Common equity securities $ 656.7 $ 69.1 $ (12.2 ) $ .2 $ 713.8 Convertible fixed maturity and preferred investments $ 3.1 $ 1.4 $ — $ — $ 4.5 Other long-term investments $ 346.0 $ 54.0 $ (11.9 ) $ 2.0 $ 390.1 December 31, 2014 Millions Cost or Gross Gross Net foreign Carrying value Common equity securities $ 633.6 $ 175.1 $ (5.2 ) $ (1.9 ) $ 801.6 Convertible fixed maturity and preferred investments $ 19.1 $ .9 $ (.2 ) $ .7 $ 20.5 Other long-term investments $ 343.3 $ 73.0 $ (10.9 ) $ 1.6 $ 407.0 Other Long-term Investments Other long-term investments consist of the following as of June 30, 2015 and December 31, 2014 : Carrying Value at Millions June 30, 2015 December 31, 2014 Hedge funds and private equity funds, at fair value (1) $ 210.8 $ 242.9 Surplus notes investments, at fair value (1) 66.3 65.1 Private equity securities and limited liability companies, at fair value (1)(2) 89.9 69.7 Tax advantaged federal affordable housing development fund (3) 15.8 16.8 Partnership investments accounted for under the equity method 3.9 5.2 Other (1) 3.4 7.3 Total other-long term investments $ 390.1 $ 407.0 (1) See Fair Value Measurements by Level table. (2) On April 2, 2015, White Mountains invested $21.0 in PassportCard. White Mountains has chosen to take the fair value election for its investment. (3) Fund accounted for using the proportional amortization method.
Other long-term investments	he following table summarizes investments in hedge funds and private equity funds by investment objective and sector as of June 30, 2015 and December 31, 2014 : June 30, 2015 December 31, 2014 Millions Fair Value Unfunded Commitments Fair Value Unfunded Commitments Hedge funds Long/short equity REIT $ 20.5 — $ 20.3 — Long/short credit & distressed 19.2 — 21.4 — Long/short equity banks and financial 15.9 $ — 29.9 $ — Other 22.1 — 20.4 — Total hedge funds 77.7 — 92.0 — Private equity funds Energy infrastructure & services 45.7 9.9 59.6 11.0 Manufacturing/Industrial 25.9 2.9 23.2 7.3 Multi-sector 24.5 4.4 24.2 5.3 Aerospace/Defense/Government 18.7 32.2 20.7 5.1 Healthcare 6.3 .8 6.1 2.8 Private equity secondaries 7.4 3.2 8.5 3.1 Insurance 2.1 41.3 2.1 41.2 Real estate 1.8 .1 3.6 3.3 Venture capital .3 — 1.4 .3 International multi-sector, Europe .4 — 1.5 2.3 Total private equity funds 133.1 94.8 150.9 81.7 Total hedge funds and private equity funds included in other long-term investments $ 210.8 $ 94.8 $ 242.9 $ 81.7
Fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds	The following summarizes the June 30, 2015 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds: Notice Period Millions Redemption frequency 30-59 days notice 60-89 days notice 90-119 days notice 120+ days notice Total Monthly $ 5.2 $ — $ — $ — $ 5.2 Quarterly 17.8 19.2 — 8.0 45.0 Semi-annual — 22.5 — — 22.5 Annual — — 4.9 .1 5.0 Total $ 23.0 $ 41.7 $ 4.9 $ 8.1 $ 77.7
Fair Value of private equity funds subject to lock-up periods	As of June 30, 2015 , investments in private equity funds were subject to lock-up periods as follows: Millions 1-3 years 3 – 5 years 5 – 10 years >10 years Total Private Equity Funds — expected lock-up period remaining $27.0 $29.5 $72.5 $4.1 $133.1
Fair value measurements by level, investment securities	The following tables summarize White Mountains’s fair value measurements for investments as of June 30, 2015 and December 31, 2014 , by major security type and class by level. The major security types were based on the legal form of the securities, with a separate break-out for convertible fixed maturity investments as they may react similar to either debt securities or equity securities, depending on prevailing market conditions. White Mountains has disaggregated its fixed maturity investments based on the issuing entity type, which impacts credit quality, with debt securities issued by U.S. government entities carrying minimal credit risk, while the credit and other risks associated with other issuers, such as corporations, foreign governments, municipalities or entities issuing asset-backed securities vary depending on the nature of the issuing entity type. White Mountains further disaggregates debt securities issued by corporations and equity securities by industry sector because investors often reference commonly used benchmarks and their subsectors to monitor risk and performance. Accordingly, White Mountains has further disaggregated these asset classes into subclasses based on the similar sectors and industry classifications it uses to evaluate investment risk and performance against commonly used benchmarks, such as the Barclays Intermediate Aggregate and S&P 500 indices: June 30, 2015 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 183.6 $ 131.5 $ 52.1 $ — Debt securities issued by corporations: Consumer 544.0 — 544.0 — Financials 405.0 — 405.0 — Health Care 306.2 — 306.2 — Industrial 281.3 — 281.3 — Communications 237.2 — 237.2 — Energy 194.2 — 194.2 — Utilities 149.0 — 149.0 — Technology 90.3 — 90.3 — Materials 98.4 — 98.4 — Other 5.5 — 5.5 — Total debt securities issued by corporations 2,311.1 — 2,311.1 — Municipal obligations 117.7 — 117.7 — Mortgage-backed and asset-backed securities 2,077.4 — 2,070.0 7.4 Foreign government, agency and provincial obligations 98.7 18.8 79.9 — Preferred stocks 86.0 — 14.8 71.2 Total fixed maturity investments 4,874.5 150.3 4,645.6 78.6 Short-term investments 840.5 838.3 2.2 — Common equity securities: Exchange traded funds (2) 361.5 340.1 21.4 — Financials 93.7 60.6 — 33.1 Consumer 51.6 51.6 — — Health Care 25.8 25.8 — — Industrial 32.3 32.3 — — Technology 38.2 38.2 — — Communications 23.8 23.8 — — Energy 2.5 2.5 — — Materials 3.6 3.6 — — Utilities 1.1 1.1 — — Other 79.7 — 79.7 — Total common equity securities 713.8 579.6 101.1 33.1 Convertible fixed maturity and preferred investments 4.5 — — 4.5 Other long-term investments (1) 370.6 — — 370.6 Total investments $ 6,803.9 $ 1,568.2 $ 4,748.9 $ 486.8 (1) Excludes carrying value of $3.9 associated with other long-term investment limited partnerships accounted for using the equity method. Excludes carrying value of $15.8 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method and $ (0.2) related to forward contracts. (2) ETFs traded on foreign exchanges are priced with an adjusted NAV and are therefore included within level 2 measurements. December 31, 2014 Millions Fair Value Level 1 Level 2 Level 3 Fixed maturity investments: U.S. Government and agency obligations $ 188.1 $ 134.1 $ 54.0 $ — Debt securities issued by corporations: Consumer 575.3 — 575.3 — Financials 431.4 — 431.4 — Health Care 284.2 — 284.2 — Industrial 225.4 — 219.8 5.6 Communications 214.3 — 214.3 — Energy 177.9 — 177.9 — Utilities 173.6 — 173.6 — Technology 118.0 — 118.0 — Materials 103.0 — 103.0 — Other 8.1 — 8.1 — Total debt securities issued by corporations 2,311.2 — 2,305.6 5.6 Municipal obligations 83.2 — 83.2 — Mortgage-backed and asset-backed securities 1,840.9 — 1,840.9 — Foreign government, agency and provincial obligations 275.1 21.3 253.8 — Preferred stocks 85.8 — 14.7 71.1 Total fixed maturity investments 4,784.3 155.4 4,552.2 76.7 Short-term investments 871.7 868.8 2.9 — Common equity securities: Financials 237.5 197.3 — 40.2 Consumer 161.2 161.1 .1 — Health Care 101.9 101.9 — — Industrial 67.5 67.5 — — Technology 54.3 54.3 — — Communications 32.9 32.9 — — Energy 32.6 32.6 — — Materials 21.2 21.2 — — Utilities 9.5 9.4 .1 — Other 83.0 9.7 73.3 — Total common equity securities 801.6 687.9 73.5 40.2 Convertible fixed maturity and preferred investments 20.5 — 12.3 8.2 Other long-term investments (1) 385.0 — — 385.0 Total investments $ 6,863.1 $ 1,712.1 $ 4,640.9 $ 510.1 (1) Excludes carrying value of $5.2 associated with other long-term investment limited partnerships accounted for using the equity method. Excludes carrying value of $16.8 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method.
Debt securities issued by corporations, credit ratings	The following table summarizes the ratings of the corporate debt securities held in White Mountains’s investment portfolio as of June 30, 2015 and December 31, 2014 : Fair Value at Millions June 30, 2015 December 31, 2014 AAA $ — $ — AA 200.5 236.9 A 887.6 957.8 BBB 1,213.5 1,105.9 BB 6.4 — Other 3.1 10.6 Debt securities issued by corporations (1) $ 2,311.1 $ 2,311.2 (1) Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor's Financial Services LLC (“Standard & Poor’s”) and 2) Moody’s Investor Service (“Moody's”).
Mortgage-backed, asset-backed securities	The following table summarizes mortgage and asset-backed securities as of June 30, 2015 and December 31, 2014 : June 30, 2015 December 31, 2014 Millions Fair Value Level 2 Level 3 Fair Value Level 2 Level 3 Mortgage-backed securities: Agency: GNMA $ 355.4 $ 355.4 $ — $ 411.2 $ 411.2 $ — FNMA 38.9 38.9 — 36.6 36.6 — FHLMC 34.8 34.8 — 49.6 49.6 — Total Agency (1) 429.1 429.1 — 497.4 497.4 — Non-agency: Residential 204.5 204.5 — 131.2 131.2 — Commercial 301.0 301.0 — 236.9 236.9 — Total Non-agency 505.5 505.5 — 368.1 368.1 — Total mortgage-backed securities 934.6 934.6 — 865.5 865.5 — Other asset-backed securities: Credit card receivables 518.6 518.6 — 522.2 522.2 — Vehicle receivables 430.0 430.0 — 289.4 289.4 — Other 194.2 186.8 7.4 163.8 163.8 — Total other asset-backed securities 1,142.8 1,135.4 7.4 975.4 975.4 — Total mortgage and asset-backed securities $ 2,077.4 $ 2,070.0 $ 7.4 $ 1,840.9 $ 1,840.9 $ — (1) Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC).
Schedule of security issuance years of investments in non-agency RMBS and non-agency CMBS securities	The security issuance years of White Mountains’s investments in non-agency residential mortgage-backed securities ("RMBS") and non-agency commercial mortgage-backed securities (“CMBS”) as of June 30, 2015 are as follows: Security Issuance Year Millions Fair Value 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 Non-agency RMBS $ 204.5 $ 33.0 $ 12.3 $ 8.7 $ — $ 14.0 $ — $ 47.4 $ 18.4 $ 4.1 $ 25.8 $ 40.8 $ — Non-agency CMBS 301.0 — — 8.3 — — — 10.7 — 25.8 58.6 121.4 76.2 Total $ 505.5 $ 33.0 $ 12.3 $ 17.0 $ — $ 14.0 $ — $ 58.1 $ 18.4 $ 29.9 $ 84.4 $ 162.2 $ 76.2
Non-agency residential mortgage securities, collateral quality and tranche levels	The classification of the underlying collateral quality and the tranche levels of White Mountains’s non-agency RMBS securities are as follows as of June 30, 2015 : Millions Fair Value Super Senior (1) Senior (2) Subordinate (3) Prime $ 202.5 $ 69.1 $ 133.4 $ — Non-prime 2.0 — 2.0 — Sub-prime — — — — Total $ 204.5 $ 69.1 $ 135.4 $ — (1) At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch Ratings (“Fitch”) and were senior to other “AAA” or “Aaa” securities. (2) At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to non-“AAA” or non-“Aaa” securities. (3) At issuance, Subordinate were not rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or "AAA" by Fitch and were junior to “AAA” or “Aaa” securities.
Non-agency commercial mortgage securities, type of interest rate and tranche levels	The amount of fixed and floating rate securities and their tranche levels of White Mountains’s non-agency CMBS securities are as follows as of June 30, 2015 : Millions Fair Value Super Senior (1) Senior (2) Subordinate (3) Fixed rate CMBS $ 204.2 $ 18.4 $ 84.5 $ 101.3 Floating rate CMBS 96.8 — — 96.8 Total $ 301.0 $ 18.4 $ 84.5 $ 198.1 (1) At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch Ratings (“Fitch”) and were senior to other “AAA” or “Aaa” securities. (2) At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or “AAA” by Fitch and were senior to non-“AAA” or non-“Aaa” securities. (3) At issuance, Subordinate were not rated “AAA” by Standard & Poor’s, “Aaa” by Moody’s or "AAA" by Fitch and were junior to “AAA” or “Aaa” securities.
Rollforward of fair value investments by level	The following tables summarize the changes in White Mountains’s fair value measurements by level for the three months ended June 30, 2015 and 2014 : Level 3 Investments Millions Level 1 investments Level 2 investments Fixed maturity investments Common equity securities Convertible fixed maturity and preferred investments Other long-term investments Total Balance at January 1, 2015 $ 843.3 $ 4,638.0 $ 76.7 $ 40.2 $ 8.2 $ 385.0 $ 5,991.4 (1)(2) Total realized and unrealized gains (losses) 13.1 31.3 — 6.9 (3.7 ) (13.3 ) 34.3 (3) Foreign currency losses through OCI (5.4 ) (62.3 ) (.2 ) — — (2.3 ) (70.2 ) Amortization/Accretion (.1 ) (21.1 ) — — — — (21.2 ) Purchases 945.9 2,002.4 76.0 — — 33.1 3,057.4 Sales (1,017.8 ) (1,915.5 ) — (9.5 ) — (18.0 ) (2,960.8 ) Effect of redemption of Prospector hedge funds (49.1 ) — — (4.5 ) — (13.9 ) (67.5 ) Transfers in — 73.9 — — — — 73.9 Transfers out — — (73.9 ) — — — (73.9 ) Balance at June 30, 2015 $ 729.9 $ 4,746.7 $ 78.6 $ 33.1 $ 4.5 $ 370.6 $ 5,963.4 (1)(2) (1) Excludes carrying value of $5.2 and $3.9 at January 1, 2015 and June 30, 2015 associated with other long-term investments accounted for using the equity method. Excludes carrying value of $16.8 and $15.8 at January 1, 2015 and June 30, 2015 million associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method. Excludes $(0.2) related to forward contracts at June 30. 2015. (2) Excludes carrying value of $ 871.7 and $ 840.5 at January 1, 2015 and June 30, 2015 associated with short-term investments. (3) Excludes $0.9 realized and unrealized losses associated with the Prospector Offshore Fund consolidation of investment-related liabilities and realized and unrealized loss for the period of $9.3 associated with short-term investments. Level 3 Investments Millions Level 1 investments Level 2 investments Fixed maturity investments Common equity securities Convertible fixed maturity and preferred investments Other long-term investments Total Balance at January 1, 2014 $ 1,376.7 $ 4,982.2 $ 93.0 $ 46.1 $ 6.1 $ 262.4 $ 6,766.5 (1)(2)(3) Total realized and unrealized gains 67.3 89.8 .9 2.7 3.2 16.8 180.7 (4) Foreign currency losses through OCI (6.8 ) (50.8 ) (.4 ) — — (1.4 ) (59.4 ) Amortization/Accretion (.3 ) (22.0 ) (.1 ) — — — (22.4 ) Purchases 953.5 1,632.0 76.1 5.0 1.5 17.4 2,685.5 Sales (896.1 ) (1,875.6 ) — — — (6.8 ) (2,778.5 ) Net change in investments consolidated affiliates (2.7 ) 7.3 — — — — 4.6 Transfers in — 19.6 8.5 — — — 28.1 Transfers out — (8.5 ) (19.6 ) — — — (28.1 ) Balance at June 30, 2014 $ 1,491.6 $ 4,774.0 $ 158.4 $ 53.8 $ 10.8 $ 288.4 $ 6,777.0 (1)(2)(3) (1) Excludes carrying value of $ 6.8 and $ 5.6 at January 1, 2014 and June 30, 2014 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) and $0.1 at January 1, 2014 and June 30, 2014 related to forward contracts. Excludes carrying value of $19.1 and $18.0 at January 1, 2014 and June 30, 2014 associated with a tax advantaged federal affordable housing development fund accounted for using the proportional amortization method. (2) Carrying value includes $236.3 and 203.8 at January 1, 2014 and June 30, 2014 that is classified as assets held for sale relating to discontinued operations. (3) Excludes carrying value of $635.9 and $ 564.0 at January 1, 2014 and June 30, 2014 associated with short-term investments. (4) Excludes $1.8 realized and unrealized losses associated with the Prospector Offshore Fund and Prospector Turtle Fund consolidation of investment-related liabilities. The Prospector Turtle Fund was liquidated as of December 31, 2014.
Schedule of significant unobservable inputs used in estimating the fair value of investment securities	The following summarizes significant unobservable inputs used in estimating the fair value of investment securities classified within Level 3 other than hedge funds and private equity funds as of June 30, 2015 and December 31, 2014. The fair value of investments in hedge funds and private equity funds, which are classified within Level 3, are estimated using the net asset value of the funds. ($ in millions) June 30, 2015 December 31, 2014 Description Rating (2) Valuation Technique(s) Fair (7) Unobservable Input Fair (7) Unobservable Input Preferred Stock (1) NR Discounted cash flow $71.2 Discount yield - 7.1% $71.1 Discount yield - 7.1% Private equity security (1) NR Multiple of GAAP book value $33.1 Book value multiple - 1.20 $40.2 Book value multiple - 1.10 Private equity security (1) NR Share price of most recent transaction $20.1 Share price - $1.06 $20.1 Share price - $1.06 Private equity security (1) NR Share price of most recent transaction $9.5 Share price - $290.96 $10.4 Share price - $290.96 Private equity security (1) NR Share price of most recent transaction $16.1 Share price - $0.13 $15.8 Share price - $0.13 Private equity security (1) NR Share price of most recent transaction $21.0 Share price - $1.00 N/A N/A N/A Convertible preferred security (1) NR Multiple of EBITDA $4.5 EBITDA multiple - 6.00 $3.8 EBITDA multiple - 6.00 Convertible preferred security (1) NR Share price of most recent transaction N/A N/A N/A $4.5 Share price - $0.71 Debt security issued by corporation (1) NR Discounted cash flow N/A N/A N/A $5.6 Illiquidity discount (3) - 10% Asset-backed security (1) A Broker pricing $7.4 Broker quote N/A N/A N/A N/A Surplus notes: NR - Seller priority Discounted cash flow $45.4 Discount rate (4) - 10.2% $44.0 Discount rate (4) - 9.3% Timing of interest payments (5) - 5 years Timing of interest payments (5) - 5 years Timing of principal payments (5) - 10 years Timing of principal payments (5) - 10 years - Pari passu Discounted cash flow $20.9 Discount rate (5) - 15.5% $21.1 Discount rate (5) - 13.5% Timing of interest payments (6) - 5 years Timing of interest payments (6) - 5 years Timing of principal payments (6) - 15 years Timing of principal payments (6) - 10 years (1) As of June 30, 2015 , consists of 1 security. (2) Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor's and 2) Moody’s. (3) Judgmentally determined based on the Company’s limited trading ability of the issuer. (4) Stochastic modeling supporting the fair value estimation indicates that the average percentage of discounted payments missed on the seller priority note is roughly equivalent to that of a conventional debt security with a credit rating of ‘B’. The corresponding credit spread increased by an additional 250 bps to reflect both a liquidity discount for a private debt instrument and regulatory payment approval uncertainty, was added to the treasury rate to determine the discount rate for the pari passu note. (5) Stochastic modeling supporting the fair value estimation indicates that the average percentage of discounted payments missed on the pari passu note is roughly equivalent to that of a conventional debt security with a credit rating of ‘CCC’. The corresponding credit spread increased by an additional 250 bps to reflect both a liquidity discount for a private debt instrument and regulatory payment approval uncertainty, was added to the treasury rate to determine the discount rate for the seller priority note. (6) For estimated value purposes, the assumption has been made that interest payouts begin in year five and principal repayments begin on a graduated basis in year ten for the seller priority note and year fifteen for the pari passu note. (7) Includes the unrealized gains and losses associated with foreign currency; foreign currency effects based on observable inputs.

Goodwill and Other Intangible30

Goodwill and Other Intangible Assets (Tables)	6 Months Ended
	Jun. 30, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets and Goodwill	The following table shows the change in goodwill and other intangible assets: Three Months Ended June 30, Millions 2015 2014 Goodwill Other intangible assets Goodwill Other intangible assets Beginning balance $ 169.2 $ 193.4 $ 23.8 $ 62.4 Acquisitions of businesses — — — — Dispositions of businesses — — — — Amortization, including foreign currency translation — (7.0 ) — (2.6 ) Ending balance $ 169.2 $ 186.4 $ 23.8 $ 59.8 Six Months Ended June 30, Millions 2015 2014 Goodwill Other intangible assets Goodwill Other intangible assets Beginning balance $ 168.9 $ 197.5 $ — $ 20.7 Acquisitions of businesses .3 2.8 23.8 42.4 Dispositions of businesses — — — (.4 ) Amortization, including foreign currency translation — (13.9 ) — (2.9 ) Ending balance $ 169.2 $ 186.4 $ 23.8 $ 59.8

Debt (Tables)

Debt (Tables)	6 Months Ended
	Jun. 30, 2015
Debt Disclosure [Abstract]
Schedule of debt outstanding	White Mountains’s debt outstanding as of June 30, 2015 and December 31, 2014 consisted of the following: Millions June 30, December 31, 2012 OBH Senior Notes, at face value $ 275.0 $ 275.0 Unamortized original issue discount (.3 ) (.3 ) 2012 OBH Senior Notes, carrying value 274.7 274.7 SIG Senior Notes, at face value 400.0 400.0 Unamortized original issue discount (.3 ) (.3 ) SIG Senior Notes, carrying value 399.7 399.7 WTM Bank Facility — — Tranzact Bank Facility 70.6 68.7 Unamortized issuance cost (1.3 ) (1.3 ) Tranzact Bank Facility, carrying value 69.3 67.4 Other debt 4.8 4.8 Total debt $ 748.5 $ 746.6

Derivatives (Tables)

Derivatives (Tables)	6 Months Ended
	Jun. 30, 2015
Derivative Instruments and Hedging Activities Disclosures [Line Items]
FOREIGN CURRENCY SWAP	Derivatives
Forward Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
FOREIGN CURRENCY SWAP	Forward Contracts White Mountains has entered into currency forward contracts at Sirius Group. White Mountains monitors its exposure to foreign currency and adjusts its forward positions within the risk guidelines and ranges established by senior management for each currency, as necessary. While White Mountains actively manages its forward positions, mismatches between movements in foreign currency rates and its forward contracts may result in currency positions being outside the pre-defined ranges and/or foreign currency losses. At June 30, 2015 , White Mountains held approximately $31.9 million (SEK 262.7 million ) total gross notional value of foreign currency forward contracts. All of White Mountains’s forward contracts are traded over-the-counter. The fair value of the contracts has been estimated using OTC quotes for similar instruments and accordingly, the measurements have been classified as Level 2 measurements at June 30, 2015 . The net realized and unrealized derivative losses recognized in net realized and unrealized investment gains (losses) for both the three and six months ended June 30, 2015 was $ 0.4 million. The net realized and unrealized derivative losses recognized in net realized and unrealized investment gains (losses) for the three and six months ended June 30, 2014 was break-even million and $(0.1) million . All of White Mountains’s forward contracts are subject to master netting agreements. As of June 30, 2015 and December 31, 2014, the gross liability amount offset under master netting arrangements and the net amount recognized in other investments approximately offset each other. White Mountains does not hold or provide any collateral for the forward contracts. The following table summarizes the notional amounts and uncollateralized balances associated with forward currency contracts: June 30, 2015 December 31, 2014 Millions Notional Amount Carrying Value Standard & Poor's Rating (1) Notional Amount Carrying Value Barclays Bank Plc $ .8 $ — A- $ 2.1 $ — Deutsche Bank 8.5 — BBB+ — — Goldman Sachs 5.0 (.1 ) AA- 8.7 — HSBC Bank Plc 12.0 (.1 ) AA- 11.2 — JP Morgan 1.2 — A 5.7 — Royal Bank of Canada 4.4 — AA- 5.4 — Total $ 31.9 $ (.2 ) $ 33.1 $ — (1) Standard & Poor's ratings as detailed above are: “AA-” (Very Strong, which is the sixth highest of twenty-three creditworthiness ratings), “A+” (Strong, which is the seventh highest of twenty-three creditworthiness ratings) and “A” (Strong, which is the eighth highest of twenty-three creditworthiness ratings).
Schedule of uncollateralized amounts due	The following table summarizes the notional amounts and uncollateralized balances associated with forward currency contracts: June 30, 2015 December 31, 2014 Millions Notional Amount Carrying Value Standard & Poor's Rating (1) Notional Amount Carrying Value Barclays Bank Plc $ .8 $ — A- $ 2.1 $ — Deutsche Bank 8.5 — BBB+ — — Goldman Sachs 5.0 (.1 ) AA- 8.7 — HSBC Bank Plc 12.0 (.1 ) AA- 11.2 — JP Morgan 1.2 — A 5.7 — Royal Bank of Canada 4.4 — AA- 5.4 — Total $ 31.9 $ (.2 ) $ 33.1 $ — (1) Standard & Poor's ratings as detailed above are: “AA-” (Very Strong, which is the sixth highest of twenty-three creditworthiness ratings), “A+” (Strong, which is the seventh highest of twenty-three creditworthiness ratings) and “A” (Strong, which is the eighth highest of twenty-three creditworthiness ratings).
Interest Rate Cap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
FOREIGN CURRENCY SWAP	Interest Rate Cap In May 2007, Sirius International Group, Ltd. (“SIG”), an intermediate holding company of Sirius Group, issued the SIG Preference Shares, with an initial fixed annual dividend rate of 7.506% . In June 2017, the fixed rate will move to a floating rate equal to the greater of (i) 7.506% and (ii) 3 -month LIBOR plus 320 basis points. In July 2013, SIG executed the Interest Rate Cap for the period from June 2017 to June 2022 to protect against a significant increase in interest rates during that 5 -year period. The Interest Rate Cap economically fixes the annual dividend rate on the SIG Preference Shares from June 2017 to June 2022 at 8.30% . The cost of the Interest Rate Cap was an upfront premium of 395 basis points of the $250.0 million notional value, or approximately $9.9 million for the full notional amount. The Interest Rate Cap does not qualify for hedge accounting. It is recorded in other assets at fair value. Changes in fair value are recognized within other revenue. Collateral held is recorded within short-term investments with an equal amount recognized as a liability to return collateral. The fair value of the Interest Rate Cap has been estimated using a single broker quote and accordingly, has been classified as a Level 3 measurement at June 30, 2015 . The following tables summarize the changes in the fair value of the Interest Rate Cap for the three and six months ended June 30, 2015 and 2014: Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Beginning of period $ 3.1 $ 8.2 $ 4.1 $ 11.1 Net realized and unrealized gains (losses) .2 (2.0 ) (.8 ) (4.9 ) End of period $ 3.3 $ 6.2 $ 3.3 $ 6.2 White Mountains does not provide any collateral to the interest rate counterparties. Under the terms of the Interest Rate Cap, White Mountains holds collateral in respect of future amounts due. White Mountains’s liability to return that collateral is based on the amounts provided by the counterparty and investment earnings thereon. The following table summarizes the Interest Rate Cap collateral balances held by White Mountains and ratings by counterparty: June 30, 2015 Millions Collateral Balances Held Standard & Poor’s Rating (1) Barclays Bank Plc $ 2.3 A- Nordea Bank Finland Plc 1.0 AA- Total $ 3.3 (1) Standard & Poor’s ratings as detailed above are: “AA-” (Very Strong, which is the fourth highest of twenty-three creditworthiness ratings) and “A-” (Strong, which is the seventh highest of twenty-three creditworthiness ratings).
Fair Value of Assets and Liabilities Measured on Recurring and Non-recurring Basis Table	The following tables summarize the changes in the fair value of the Interest Rate Cap for the three and six months ended June 30, 2015 and 2014: Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Beginning of period $ 3.1 $ 8.2 $ 4.1 $ 11.1 Net realized and unrealized gains (losses) .2 (2.0 ) (.8 ) (4.9 ) End of period $ 3.3 $ 6.2 $ 3.3 $ 6.2
Variable Annuity
Derivative Instruments and Hedging Activities Disclosures [Line Items]
FOREIGN CURRENCY SWAP	Variable Annuity Reinsurance White Mountains has entered into agreements to reinsure death and living benefit guarantees associated with certain variable annuities in Japan. At June 30, 2015 and December 31, 2014 , the total guarantee value was approximately ¥ 100.7 billion (approximately $0.8 billion at exchange rates on that date) and ¥134.2 billion (approximately $1.1 billion at exchange rates on that date), respectively. The collective account values of the underlying variable annuities were approximately 113% of the guarantee value at both June 30, 2015 and December 31, 2014 . During the second quarter of 2015, the variable annuity contracts reinsured by WM Life Re began to mature. WMLife Re is in runoff, an all of its contracts will mature by June 30, 2016. The following table summarizes the pre-tax operating results of WM Life Re for the three and six months ended June 30, 2015 and 2014 . Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Fees, included in other revenue $ 2.6 $ 5.1 $ 5.6 $ 10.5 Change in fair value of variable annuity liability, included in other revenue .7 22.0 1.1 20.7 Change in fair value of derivatives, included in other revenue (2.3 ) (26.2 ) (7.8 ) (32.7 ) Foreign exchange, included in other revenue — .2 (1.2 ) .5 Other investment income and (losses) gains (.1 ) .2 (.2 ) .5 Total revenue .9 1.3 (2.5 ) (.5 ) Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses — .3 — .3 Death benefit claims paid, included in general and administrative expenses (.1 ) (.1 ) — (.1 ) General and administrative expenses (.9 ) (1.0 ) (2.3 ) (2.4 ) Pre-tax loss $ (.1 ) $ .5 $ (4.8 ) $ (2.7 ) The following summarizes realized and unrealized derivative gains (losses) recognized in other revenue for the three and six months ended June 30, 2015 and 2014 and the carrying values, included in other assets, at June 30, 2015 and December 31, 2014 by type of instrument: Gains (losses) Carrying Value Three Months Ended Six Months Ended As of June 30, June 30, June 30, December 31, 2014 Millions 2015 2014 2015 2014 Fixed income/interest rate $ (5.9 ) $ (5.6 ) $ 3.4 $ (12.0 ) $ (.1 ) $ (1.7 ) Foreign exchange 9.2 (6.4 ) (2.9 ) (12.9 ) 29.0 44.1 Equity (5.6 ) (14.2 ) (8.3 ) (7.8 ) 6.8 14.0 Total $ (2.3 ) $ (26.2 ) $ (7.8 ) $ (32.7 ) $ 35.7 $ 56.4 The following tables summarize the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the three and six months ended June 30, 2015 and 2014 : Three Months Ended June 30, 2015 Variable Annuity Assets Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ 1.1 $ 17.0 $ 31.2 $ (1.2 ) $ 47.0 Purchases — — — — — Realized and unrealized gains (losses) .7 (8.3 ) .6 5.4 (2.3 ) Transfers in — — — — — Sales/settlements — (.4 ) .7 (9.3 ) (9.0 ) End of period $ 1.8 $ 8.3 $ 32.5 $ (5.1 ) $ 35.7 Six Months Ended June 30, 2015 Variable Annuity Assets Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ .7 $ 18.9 $ 33.8 $ 3.7 $ 56.4 Purchases — — — — — Realized and unrealized gains (losses) 1.1 (9.0 ) (5.9 ) 7.1 (7.8 ) Transfers in — — — — — Sales/settlements — (1.6 ) 4.6 (15.9 ) (12.9 ) End of period $ 1.8 $ 8.3 $ 32.5 $ (5.1 ) $ 35.7 Three Months Ended June 30, 2014 Variable Annuity Assets Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ (54.1 ) $ 64.1 $ 25.1 $ .4 $ 89.6 Purchases — — — — — Realized and unrealized gains (losses) 22.3 (10.6 ) (15.5 ) (.1 ) (26.2 ) Transfers in — — — — — Sales/settlements — (.2 ) 12.7 (1.1 ) 11.4 End of period $ (31.8 ) $ 53.3 $ 22.3 $ (.8 ) $ 74.8 Six Months Ended June 30, 2014 Variable Annuity (Liabilities) Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ (52.8 ) $ 63.4 $ 4.7 $ 1.1 $ 69.2 Purchases — — — — — Realized and unrealized gains (losses) 21.0 (9.9 ) (22.5 ) (.3 ) (32.7 ) Transfers in — — — — — Sales/settlements — (.2 ) 40.1 (1.6 ) 38.3 End of period $ (31.8 ) $ 53.3 $ 22.3 $ (.8 ) $ 74.8 (1) Consists of over-the-counter instruments. (2) Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available. Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index. (3) Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded. In addition to derivative instruments, WM Life Re held cash and fixed maturity investments posted as collateral to its variable annuity reinsurance counterparties. The total collateral includes the following: Millions June 30, 2015 December 31, 2014 June 30, 2014 Cash $ 20.9 $ 23.7 $ 27.8 Fixed maturity investments 4.4 9.5 14.5 Total $ 25.3 $ 33.2 $ 42.3 Collateral in the form of fixed maturity securities consists of Government of Japan Bonds, which are recorded at fair value. Collateral in the form of short-term investments consists of money-market instruments, carried at amortized cost, which approximates fair value. All of White Mountains’s variable annuity reinsurance liabilities were classified as Level 3 measurements at June 30, 2015 and 2014 . The fair value of White Mountains’s variable annuity reinsurance liabilities are estimated using actuarial and capital market assumptions related to the projected discounted cash flows over the term of the reinsurance agreement. Actuarial assumptions regarding future policyholder behavior, including surrender and lapse rates, are generally unobservable inputs and significantly impact the fair value estimates. Market conditions including, but not limited to, changes in interest rates, equity indices, market volatility and foreign currency exchange rates as well as the variations in actuarial assumptions regarding policyholder behavior may result in significant fluctuations in the fair value estimates. Generally, the liabilities associated with these guarantees increase with declines in the equity markets, interest rates and currencies against the Japanese yen, as well as with increases in market volatilities. White Mountains uses derivative instruments, including put options, interest rate swaps, total return swaps on bond and equity indices and forwards and futures contracts on major equity indices, currency pairs and government bonds, to mitigate the risks associated with changes in the fair value of the reinsured variable annuity guarantees. The types of inputs used to estimate the fair value of these derivative instruments, with the exception of actuarial assumptions regarding policyholder behavior and risk margins, are generally the same as those used to estimate the fair value of variable annuity liabilities. The following summarizes quantitative information about significant unobservable inputs associated with the fair value estimates for variable annuity reinsurance liabilities and derivative instruments that have been classified as Level 3 measurements: ($ in Millions) June 30, 2015 Description Fair Value Valuation Technique(s) Unobservable Input Range Weighted Average Variable annuity benefit guarantee liabilities $ (1.8 ) Discounted cash flows Surrenders 0-1 year 0.1 % - 40.0% 40.0 % Mortality 0.0 % - 6.4% 1.1 % Foreign exchange volatilities 0-1 year 10.6 % - 16.1% 12.8 % Index volatilities 0-1 year 24.5 % - 27.9% 26.3 % Foreign exchange options $ 2.7 Counterparty valuations, adjusted for unwind quote discount Adjustment to counterparty valuations 0.5 % - 11.6% 5.3 % Equity index options $ 5.6 Counterparty valuations, adjusted for unwind quote discount Adjustment to counterparty valuations (0.7 )% - 6.5% 1.5 % WM Life Re enters into both over-the-counter (“OTC”) and exchange traded derivative instruments to economically hedge the liability from the variable annuity benefit guarantee. In the case of OTC derivatives, WM Life Re has exposure to credit risk for amounts that are uncollateralized by counterparties. WM Life Re’s internal risk management guidelines establish net counterparty exposure thresholds that take into account OTC counterparties’ credit ratings. The OTC derivative contracts are subject to restrictions on liquidation of the instruments and distribution of proceeds under collateral agreements. In the case of exchange traded instruments, WM Life Re has exposure to credit risk for amounts uncollateralized by margin balances. WM Life Re has entered into master netting agreements with certain of its counterparties whereby the collateral provided (held) is calculated on a net basis. The following summarizes amounts offset under master netting agreements: June 30, 2015 December 31, 2014 Millions Gross asset amounts before offsets (1) Gross liability amounts offset under master netting arrangements Net amounts recognized in Other Assets Gross asset amounts before offsets (1) Gross liability amounts offset under master netting arrangements Net amounts recognized in Other Assets Interest rate contracts OTC $ 2.2 $ (2.1 ) $ 0.1 $ 1.0 $ (5.4 ) $ (4.4 ) Exchange traded .2 (.6 ) (.4 ) 2.8 (.1 ) 2.7 Foreign exchange contracts OTC 31.6 — 31.6 45.5 — 45.5 Exchange traded — (2.3 ) (2.3 ) — (1.4 ) (1.4 ) Equity contracts OTC 10.2 (1.2 ) 9.0 11.7 (.2 ) 11.5 Exchange traded — (2.2 ) (2.2 ) 3.4 (.9 ) 2.5 Total (2) $ 44.2 $ (8.4 ) $ 35.8 $ 64.4 $ (8.0 ) $ 56.4 (1) Amount equal to fair value of instrument as recognized in other assets (2) All derivative instruments held by WM Life Re are subject to master netting arrangements. The following summarizes the value, collateral held or provided by WM Life Re and net exposure to credit losses on OTC and exchange traded derivative instruments by counterparty recorded within other assets: June 30, 2015 Millions Net amount of assets reflected in Balance Sheet Collateral provided to counterparty - Cash Collateral provided to counter-party - Financial Instruments Net amount of exposure after effect of collateral provided Excess collateral provided to counter-party- Cash Excess collateral provided - Financial Instruments Counter-party collateral held by WMLife Re - Cash Net amount of exposure to counter-party Standard & Poor's Rating (1) Bank of America $ 2.0 $ — $ — $ 2.0 $ — $ — $ — $ 2.0 A Barclays — — — — — — — — A JP Morgan 16.8 — — 16.8 — — 6.5 10.3 A + Royal Bank of Scotland 1.0 — — 1.0 — — — 1.0 A - Nomura (.3 ) .3 — — 4.9 4.4 — 9.3 BBB + Citigroup - OTC 21.3 — — 21.3 — — 4.9 16.4 A Citigroup - Exchange Traded (5.0 ) 5.0 — — 10.7 — — 10.7 A Total $ 35.8 $ 5.3 $ — $ 41.1 $ 15.6 $ 4.4 $ 11.4 $ 49.7 December 31, 2014 Millions Net amount of assets reflected in Balance Sheet Collateral provided to counter-party - Cash Collateral provided to counter-party - Financial Instruments Net amount of exposure after effect of collateral provided Excess collateral provided to counter-party- Cash Excess collateral provided - Financial Instruments Counter-party collateral held by WMLife Re- Cash Net amount of exposure to counter-party Standard & Poor's Rating (1) Bank of America $ 5.6 $ — $ — $ 5.6 $ — $ — $ — $ 5.6 A Barclays .1 — — .1 — — — .1 A JP Morgan 24.3 — — 24.3 — — 8.8 15.5 A + Royal Bank of Scotland 4.0 — — 4.0 — — — 4.0 A Nomura (3.5 ) 3.5 — — 1.7 9.5 — 11.2 BBB + Citigroup - OTC 22.2 — — 22.2 — — 1.1 21.1 A Citigroup - Exchange Traded 3.7 — — 3.7 16.0 — — 19.7 A Total $ 56.4 $ 3.5 $ — $ 59.9 $ 17.7 $ 9.5 $ 9.9 $ 77.2 (1) Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-three creditworthiness ratings), “A” (Strong, which is the sixth highest of twenty-three creditworthiness ratings), “A-” (Strong, which is the seventh highest of twenty-three creditworthiness ratings) and “BBB+” (Adequate, which is the eighth highest of twenty-three creditworthiness ratings).
Pre-tax operating results of WM Life Re	The following table summarizes the pre-tax operating results of WM Life Re for the three and six months ended June 30, 2015 and 2014 . Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Fees, included in other revenue $ 2.6 $ 5.1 $ 5.6 $ 10.5 Change in fair value of variable annuity liability, included in other revenue .7 22.0 1.1 20.7 Change in fair value of derivatives, included in other revenue (2.3 ) (26.2 ) (7.8 ) (32.7 ) Foreign exchange, included in other revenue — .2 (1.2 ) .5 Other investment income and (losses) gains (.1 ) .2 (.2 ) .5 Total revenue .9 1.3 (2.5 ) (.5 ) Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses — .3 — .3 Death benefit claims paid, included in general and administrative expenses (.1 ) (.1 ) — (.1 ) General and administrative expenses (.9 ) (1.0 ) (2.3 ) (2.4 ) Pre-tax loss $ (.1 ) $ .5 $ (4.8 ) $ (2.7 )
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values, by the type of instrument	The following summarizes realized and unrealized derivative gains (losses) recognized in other revenue for the three and six months ended June 30, 2015 and 2014 and the carrying values, included in other assets, at June 30, 2015 and December 31, 2014 by type of instrument: Gains (losses) Carrying Value Three Months Ended Six Months Ended As of June 30, June 30, June 30, December 31, 2014 Millions 2015 2014 2015 2014 Fixed income/interest rate $ (5.9 ) $ (5.6 ) $ 3.4 $ (12.0 ) $ (.1 ) $ (1.7 ) Foreign exchange 9.2 (6.4 ) (2.9 ) (12.9 ) 29.0 44.1 Equity (5.6 ) (14.2 ) (8.3 ) (7.8 ) 6.8 14.0 Total $ (2.3 ) $ (26.2 ) $ (7.8 ) $ (32.7 ) $ 35.7 $ 56.4
Fair Value of Assets and Liabilities Measured on Recurring and Non-recurring Basis Table	The following tables summarize the changes in White Mountains’s variable annuity reinsurance liabilities and derivative instruments for the three and six months ended June 30, 2015 and 2014 : Three Months Ended June 30, 2015 Variable Annuity Assets Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ 1.1 $ 17.0 $ 31.2 $ (1.2 ) $ 47.0 Purchases — — — — — Realized and unrealized gains (losses) .7 (8.3 ) .6 5.4 (2.3 ) Transfers in — — — — — Sales/settlements — (.4 ) .7 (9.3 ) (9.0 ) End of period $ 1.8 $ 8.3 $ 32.5 $ (5.1 ) $ 35.7 Six Months Ended June 30, 2015 Variable Annuity Assets Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ .7 $ 18.9 $ 33.8 $ 3.7 $ 56.4 Purchases — — — — — Realized and unrealized gains (losses) 1.1 (9.0 ) (5.9 ) 7.1 (7.8 ) Transfers in — — — — — Sales/settlements — (1.6 ) 4.6 (15.9 ) (12.9 ) End of period $ 1.8 $ 8.3 $ 32.5 $ (5.1 ) $ 35.7 Three Months Ended June 30, 2014 Variable Annuity Assets Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ (54.1 ) $ 64.1 $ 25.1 $ .4 $ 89.6 Purchases — — — — — Realized and unrealized gains (losses) 22.3 (10.6 ) (15.5 ) (.1 ) (26.2 ) Transfers in — — — — — Sales/settlements — (.2 ) 12.7 (1.1 ) 11.4 End of period $ (31.8 ) $ 53.3 $ 22.3 $ (.8 ) $ 74.8 Six Months Ended June 30, 2014 Variable Annuity (Liabilities) Derivative Instruments Millions Level 3 Level 3 (1) Level 2 (1)(2) Level 1 (3) Total Beginning of period $ (52.8 ) $ 63.4 $ 4.7 $ 1.1 $ 69.2 Purchases — — — — — Realized and unrealized gains (losses) 21.0 (9.9 ) (22.5 ) (.3 ) (32.7 ) Transfers in — — — — — Sales/settlements — (.2 ) 40.1 (1.6 ) 38.3 End of period $ (31.8 ) $ 53.3 $ 22.3 $ (.8 ) $ 74.8 (1) Consists of over-the-counter instruments. (2) Consists of interest rate swaps, total return swaps, foreign currency forward contracts, and bond forwards. Fair value measurement based upon bid/ask pricing quotes for similar instruments that are actively traded, where available. Swaps for which an active market does not exist have been priced using observable inputs including the swap curve and the underlying bond index. (3) Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.
Assets Held under Reinsurance Counterparty Trust Agreement	In addition to derivative instruments, WM Life Re held cash and fixed maturity investments posted as collateral to its variable annuity reinsurance counterparties. The total collateral includes the following: Millions June 30, 2015 December 31, 2014 June 30, 2014 Cash $ 20.9 $ 23.7 $ 27.8 Fixed maturity investments 4.4 9.5 14.5 Total $ 25.3 $ 33.2 $ 42.3
Schedule of significant unobservable inputs associated with the fair value estimates for variable annuity reinsurance liabilities and derivative instruments	The following summarizes quantitative information about significant unobservable inputs associated with the fair value estimates for variable annuity reinsurance liabilities and derivative instruments that have been classified as Level 3 measurements: ($ in Millions) June 30, 2015 Description Fair Value Valuation Technique(s) Unobservable Input Range Weighted Average Variable annuity benefit guarantee liabilities $ (1.8 ) Discounted cash flows Surrenders 0-1 year 0.1 % - 40.0% 40.0 % Mortality 0.0 % - 6.4% 1.1 % Foreign exchange volatilities 0-1 year 10.6 % - 16.1% 12.8 % Index volatilities 0-1 year 24.5 % - 27.9% 26.3 % Foreign exchange options $ 2.7 Counterparty valuations, adjusted for unwind quote discount Adjustment to counterparty valuations 0.5 % - 11.6% 5.3 % Equity index options $ 5.6 Counterparty valuations, adjusted for unwind quote discount Adjustment to counterparty valuations (0.7 )% - 6.5% 1.5 %
Exchange traded and OTC derivative instruments	The following summarizes amounts offset under master netting agreements: June 30, 2015 December 31, 2014 Millions Gross asset amounts before offsets (1) Gross liability amounts offset under master netting arrangements Net amounts recognized in Other Assets Gross asset amounts before offsets (1) Gross liability amounts offset under master netting arrangements Net amounts recognized in Other Assets Interest rate contracts OTC $ 2.2 $ (2.1 ) $ 0.1 $ 1.0 $ (5.4 ) $ (4.4 ) Exchange traded .2 (.6 ) (.4 ) 2.8 (.1 ) 2.7 Foreign exchange contracts OTC 31.6 — 31.6 45.5 — 45.5 Exchange traded — (2.3 ) (2.3 ) — (1.4 ) (1.4 ) Equity contracts OTC 10.2 (1.2 ) 9.0 11.7 (.2 ) 11.5 Exchange traded — (2.2 ) (2.2 ) 3.4 (.9 ) 2.5 Total (2) $ 44.2 $ (8.4 ) $ 35.8 $ 64.4 $ (8.0 ) $ 56.4 (1) Amount equal to fair value of instrument as recognized in other assets (2) All derivative instruments held by WM Life Re are subject to master netting arrangements.
Net exposure to collateral	The following summarizes the value, collateral held or provided by WM Life Re and net exposure to credit losses on OTC and exchange traded derivative instruments by counterparty recorded within other assets: June 30, 2015 Millions Net amount of assets reflected in Balance Sheet Collateral provided to counterparty - Cash Collateral provided to counter-party - Financial Instruments Net amount of exposure after effect of collateral provided Excess collateral provided to counter-party- Cash Excess collateral provided - Financial Instruments Counter-party collateral held by WMLife Re - Cash Net amount of exposure to counter-party Standard & Poor's Rating (1) Bank of America $ 2.0 $ — $ — $ 2.0 $ — $ — $ — $ 2.0 A Barclays — — — — — — — — A JP Morgan 16.8 — — 16.8 — — 6.5 10.3 A + Royal Bank of Scotland 1.0 — — 1.0 — — — 1.0 A - Nomura (.3 ) .3 — — 4.9 4.4 — 9.3 BBB + Citigroup - OTC 21.3 — — 21.3 — — 4.9 16.4 A Citigroup - Exchange Traded (5.0 ) 5.0 — — 10.7 — — 10.7 A Total $ 35.8 $ 5.3 $ — $ 41.1 $ 15.6 $ 4.4 $ 11.4 $ 49.7 December 31, 2014 Millions Net amount of assets reflected in Balance Sheet Collateral provided to counter-party - Cash Collateral provided to counter-party - Financial Instruments Net amount of exposure after effect of collateral provided Excess collateral provided to counter-party- Cash Excess collateral provided - Financial Instruments Counter-party collateral held by WMLife Re- Cash Net amount of exposure to counter-party Standard & Poor's Rating (1) Bank of America $ 5.6 $ — $ — $ 5.6 $ — $ — $ — $ 5.6 A Barclays .1 — — .1 — — — .1 A JP Morgan 24.3 — — 24.3 — — 8.8 15.5 A + Royal Bank of Scotland 4.0 — — 4.0 — — — 4.0 A Nomura (3.5 ) 3.5 — — 1.7 9.5 — 11.2 BBB + Citigroup - OTC 22.2 — — 22.2 — — 1.1 21.1 A Citigroup - Exchange Traded 3.7 — — 3.7 16.0 — — 19.7 A Total $ 56.4 $ 3.5 $ — $ 59.9 $ 17.7 $ 9.5 $ 9.9 $ 77.2 (1) Standard & Poor’s ratings as detailed above are: “A+” (Strong, which is the fifth highest of twenty-three creditworthiness ratings), “A” (Strong, which is the sixth highest of twenty-three creditworthiness ratings), “A-” (Strong, which is the seventh highest of twenty-three creditworthiness ratings) and “BBB+” (Adequate, which is the eighth highest of twenty-three creditworthiness ratings).
Forward Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
FOREIGN CURRENCY SWAP	Foreign Currency Swap On April 28, 2015, White Mountains executed two foreign currency swaps, each with a notional amount of $50.0 million , maturing on March 20, 2017. Under the first swap, White Mountains pays Swedish krona and receives U.S. dollars. Under the second swap, White Mountains pays Euro and receives U.S. dollars. The swaps, which were executed as part of White Mountains's management of overall foreign currency exposure at Sirius Group, have not been designated or accounted for under hedge accounting. At June 30, 2015, the fair value of the swaps of $(2.7) million was recorded within other assets. Changes in fair value are recognized as unrealized gains or losses and are presented within other revenues. White Mountains does not provide or hold any collateral associated with the swaps.

Municipal Bond Guarantee (Table

Municipal Bond Guarantee (Tables)	6 Months Ended
	Jun. 30, 2015
Guarantees [Abstract]
Schedule of Municipal Bond Guarantee Insured Obligations	The following table provides a schedule of BAM’s insured obligations: June 30, 2015 December 31, 2014 Contracts outstanding 2,436 1,750 Remaining weighted average contract period outstanding (in years) 12.7 12.8 Contractual debt service outstanding (in millions): Par $ 17,766.6 $ 12,362.5 Interest 9,588.1 7,086.9 Total debt service outstanding $ 27,354.7 $ 19,449.4 Gross unearned insurance premiums $ 37.1 $ 27.6

Earnings Per Share (Tables)

Earnings Per Share (Tables)	6 Months Ended
	Jun. 30, 2015
Earnings Per Share [Abstract]
Computation of earnings per share	The following table outlines the Company’s computation of earnings per share from continuing operations for the three and six months ended June 30, 2015 and 2014 . (See Note 17 - “Discontinued Operations” ). Three Months Ended Six Months Ended June 30, June 30, 2015 2014 2015 2014 Basic and diluted earnings per share numerators (in millions): Net income from continuing operations attributable to White Mountains’s common shareholders $ 4.6 $ 92.9 $ 81.0 $ 188.9 Allocation of income for unvested restricted common shares (.1 ) (1.2 ) (.9 ) (2.3 ) Dividends declared on participating restricted common shares (1) — — (.1 ) (.1 ) Total allocation to restricted common shares (.1 ) (1.2 ) (1.0 ) (2.4 ) Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 4.5 $ 91.7 $ 80.0 $ 186.5 Undistributed net earnings (in millions): Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 4.5 $ 91.7 $ 80.0 $ 186.5 Dividends declared net of restricted common share amounts (1) — — (5.9 ) (6.1 ) Total undistributed net earnings, net of restricted common share amounts $ 4.5 $ 91.7 $ 74.1 $ 180.4 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 5,985.7 6,162.2 5,982.1 6,166.0 Average unvested restricted shares (2) (71.4 ) (81.3 ) (65.2 ) (75.9 ) Basic earnings per share denominator 5,914.3 6,080.9 5,916.9 6,090.1 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 5,985.7 6,162.2 5,982.1 6,166.0 Average unvested restricted common shares (2) (71.4 ) (81.3 ) (65.2 ) (75.9 ) Average outstanding dilutive options to acquire common shares (3) — — — — Diluted earnings per share denominator 5,914.3 6,080.9 5,916.9 6,090.1 Basic earnings per share (in dollars): Net income attributable to White Mountains’s common shareholders $ .76 $ 15.08 $ 13.53 $ 30.61 Dividends declared and paid — — (1.00 ) (1.00 ) Undistributed earnings $ .76 $ 15.08 $ 12.53 $ 29.61 Diluted earnings per share (in dollars): Net income attributable to White Mountains’s common shareholders $ .76 $ 15.08 $ 13.53 $ 30.61 Dividends declared and paid — — (1.00 ) (1.00 ) Undistributed earnings $ .76 $ 15.08 $ 12.53 $ 29.61 (1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (2) Restricted shares outstanding vest either in equal annual installments or upon a stated date. (See Note 15 - “Employee Share-Based Compensation Plans”) . (3) The diluted earnings per share denominator for the three and six months ended June 30, 2015 and 2014 does not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.

Non-controlling Interest (Table

Non-controlling Interest (Tables)	6 Months Ended
	Jun. 30, 2015
Noncontrolling Interest [Line Items]
Noncontrolling Interest Disclosure [Text Block]	The following table details the balance of non-controlling interests included in White Mountains’s total equity and the related percentage of each consolidated entity’s total equity owned by non-controlling shareholders as of June 30, 2015 and December 31, 2014: June 30, 2015 December 31, 2014 $ in millions Non-controlling Percentage Non-controlling Equity Non-controlling Percentage Non-controlling Equity OneBeacon Ltd. 24.7 % $ 255.0 24.7 % $ 258.4 SIG Preference Shares 100.0 250.0 100.0 250.0 Other, excluding mutuals and reciprocals HG Global 3.1 17.5 3.1 17.9 MediaAlpha 40.0 21.5 40.0 22.6 Tranzact 36.8 80.3 36.8 88.2 Wobi 4.7 2.3 36.7 5.4 Dewar 31.9 3.5 18.0 3.4 Prospector Offshore Fund — — 23.4 31.1 Total other, excluding mutuals and reciprocals 125.1 168.6 Mutuals and reciprocals BAM 100.0 (131.8 ) 100.0 (121.9 ) SSIE 100.0 (14.4 ) 100.0 (12.4 ) Total non-controlling interests $ 483.9 $ 542.7

Segment Information (Tables)

Segment Information (Tables)	6 Months Ended
	Jun. 30, 2015
Segment Reporting [Abstract]
Financial information for White Mountains' segments	Significant intercompany transactions among White Mountains’s segments have been eliminated herein. Financial information for White Mountains’s segments follows: HG Global/BAM Millions OneBeacon Sirius Group HG BAM Other Operations Total Three Months Ended June 30, 2015 Earned insurance and reinsurance premiums $ 319.3 $ 215.5 $ .6 $ .1 $ 1.8 $ 537.3 Net investment income 10.1 9.3 .5 .8 3.1 23.8 Net investment income (loss) - surplus note interest — — 3.9 (3.9 ) — — Net realized and unrealized investment losses (14.0 ) (42.7 ) (1.4 ) (1.9 ) (1.1 ) (61.1 ) Other (loss) revenue (1.2 ) 5.4 — .2 83.1 87.5 Total revenues 314.2 187.5 3.6 (4.7 ) 86.9 587.5 Losses and LAE 194.5 92.1 — — 1.7 288.3 Insurance and reinsurance acquisition expenses 56.4 47.5 .1 .6 .7 105.3 Other underwriting expenses 52.9 27.7 — .1 — 80.7 General and administrative expenses 4.2 6.8 .4 8.7 88.0 108.1 Interest expense 3.3 6.6 — — 1.0 10.9 Total expenses 311.3 180.7 .5 9.4 91.4 593.3 Pre-tax income (loss) $ 2.9 $ 6.8 $ 3.1 $ (14.1 ) $ (4.5 ) $ (5.8 ) HG Global/BAM Millions OneBeacon Sirius Group HG BAM Other Operations Total Six Months Ended June 30, 2015 Earned insurance and reinsurance premiums $ 605.9 $ 420.5 $ 1.1 $ .3 $ 3.8 $ 1,031.6 Net investment income 20.6 17.3 .9 1.8 6.5 47.1 Net investment income (loss) - surplus note interest — — 7.9 (7.9 ) — — Net realized and unrealized investment gains (loses) .2 37.2 (.3 ) 1.1 4.1 42.3 Other revenue (loss) (4.3 ) (22.0 ) — .3 149.3 123.3 Total revenues 622.4 453.0 9.6 (4.4 ) 163.7 1,244.3 Losses and LAE 360.4 189.6 — — 3.7 553.7 Insurance and reinsurance acquisition expenses 107.4 90.1 .2 1.4 1.8 200.9 Other underwriting expenses 108.8 53.2 — .2 — 162.2 General and administrative expenses 8.3 13.0 .8 16.9 185.5 224.5 Interest expense 6.5 13.4 — — 2.1 22.0 Total expenses 591.4 359.3 1.0 18.5 193.1 1,163.3 Pre-tax income (loss) $ 31.0 $ 93.7 $ 8.6 $ (22.9 ) $ (29.4 ) $ 81.0 HG Global/BAM Millions OneBeacon Sirius Group HG BAM Other Operations Total Three Months Ended June 30, 2014 Earned insurance and reinsurance premiums $ 290.9 $ 209.6 $ .3 $ .1 $ 2.5 $ 503.4 Net investment income 12.2 11.8 .3 1.4 3.9 29.6 Net investment income (loss) - surplus note interest — — 4.0 (4.0 ) — — Net realized and unrealized investment gains 21.9 58.4 .9 3.8 28.9 113.9 Other revenue (loss) 1.0 (19.2 ) — .1 24.5 6.4 Total revenues 326.0 260.6 5.5 1.4 59.8 653.3 Losses and LAE 186.1 91.4 — — 5.2 282.7 Insurance and reinsurance acquisition expenses 49.6 48.8 .1 .5 (.1 ) 98.9 Other underwriting expenses 51.7 30.2 — .1 (.1 ) 81.9 General and administrative expenses 3.6 6.9 .4 8.8 50.6 70.3 Interest expense 3.3 6.6 — — .1 10.0 Total expenses 294.3 183.9 .5 9.4 55.7 543.8 Pre-tax income (loss) $ 31.7 $ 76.7 $ 5.0 $ (8.0 ) $ 4.1 $ 109.5 HG Global/BAM Millions OneBeacon Sirius Group HG BAM Other Operations Total Six Months Ended June 30, 2014 Earned insurance and reinsurance premiums $ 567.4 $ 425.1 $ .5 $ .2 $ 3.8 $ 997.0 Net investment income 22.6 20.5 .6 2.8 7.8 54.3 Net investment income (loss) - surplus note interest — — 7.9 (7.9 ) — — Net realized and unrealized investment gains 40.8 86.5 1.6 6.8 42.0 177.7 Other revenue (loss) 2.0 (24.5 ) — .3 25.2 3.0 Total revenues 632.8 507.6 10.6 2.2 78.8 1,232.0 Losses and LAE 335.5 170.2 — — 6.3 512.0 Insurance and reinsurance acquisition expenses 96.3 96.2 .2 .9 .4 194.0 Other underwriting expenses 101.1 62.0 — .2 — 163.3 General and administrative expenses 6.9 15.3 .8 17.7 79.9 120.6 Interest expense 6.5 13.2 — — .4 20.1 Total expenses 546.3 356.9 1.0 18.8 87.0 1,010.0 Pre-tax income (loss) $ 86.5 $ 150.7 $ 9.6 $ (16.6 ) $ (8.2 ) $ 222.0

Investments in Unconsolidated37

Investments in Unconsolidated Affiliates (Tables)	6 Months Ended
	Jun. 30, 2015
Equity Method Investments and Joint Ventures [Abstract]
Investments in and Advances to Affiliates	White Mountains’s investments in unconsolidated affiliates represent investments in other companies in which White Mountains has a significant voting and economic interest but does not control the entity. Millions June 30, December 31, Symetra common shares $ 395.2 $ 373.8 Unrealized gains from Symetra’s fixed maturity portfolio 2.0 37.6 Carrying value of Symetra common shares 397.2 411.4 Hamer — 3.0 Total investments in unconsolidated affiliates $ 397.2 $ 414.4
Schedule of Changes in Investments in Unconsolidated Affiliates	The following table summarizes amounts recorded by White Mountains relating to its investment in Symetra for the three and six months ended June 30, 2015 and 2014 : Three Months Ended Six Months Ended June 30, June 30, Millions 2015 2014 2015 2014 Carrying value of investment in Symetra at beginning of period $ 447.3 $ 369.0 $ 411.4 $ 317.3 Equity in earnings (1)(2) 6.1 13.1 13.3 27.5 Equity in net unrealized (losses) gains from Symetra’s fixed maturity portfolio (3) (66.4 ) 35.4 (35.5 ) 74.7 Dividends received (2.2 ) (2.0 ) (4.4 ) (4.0 ) Distribution from Prospector Offshore Fund 12.4 — 12.4 — Carrying value of investment in Symetra at end of period (4)(5) $ 397.2 $ 415.5 $ 397.2 $ 415.5 (1) For the three months ended June 30, 2015 and 2014 , equity in earnings excludes tax expense of $0.4 and $0.9 . For the six months ended June 30, 2015 and 2014 , equity in earnings excludes tax expense of $0.8 and $1.9 . (2) For the three months ended June 30, 2015 and 2014 , equity in earnings includes $0.7 and $0.7 increase relating to the pre-tax amortization of the Symetra common share basis difference. For the six months ended June 30, 2015 and 2014 , equity in earnings includes $1.4 and $1.5 increase relating to the pre-tax amortization of the Symetra common share basis difference. (3) For the three months ended June 30, 2015 and 2014 , net unrealized gains includes $2.8 and $3.1 increase relating to the pre-tax amortization of the Symetra common share basis difference. For the six months ended June 30, 2015 and 2014 , net unrealized gains includes $5.7 and $6.3 increase relating to the pre-tax amortization of the Symetra common share basis difference. (4) Includes White Mountains’s equity in net unrealized gains from Symetra’s fixed maturity portfolio of $2.0 and $31.1 as of June 30, 2015 and 2014 , which excludes tax expense of $0.0 and $2.1 . (5) The aggregate value of White Mountains’s investment in common shares of Symetra was $497.0 based upon the quoted market price of $ 24.17 per share at June 30, 2015 .

Employee Share-Based Incentiv38

Employee Share-Based Incentive Compensation Plans (Tables)	6 Months Ended
	Jun. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of performance share activity	The following table summarizes performance share activity for the three and six months ended June 30, 2015 and 2014 for performance shares granted under the WTM Incentive Plan: Three Months Ended June 30, Six Months Ended June 30, 2015 2014 2015 2014 Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 116,396 $ 27.6 127,537 $ 42.3 123,549 $ 48.1 119,220 $ 60.2 Shares paid or expired (1) — — — — (37,977 ) (27.3 ) (37,130 ) (26.7 ) New grants 750 — — — 31,390 — 45,660 — Assumed forfeitures and cancellations (2) (19 ) (.1 ) — (.2 ) 165 .5 (213 ) .2 Expense recognized — 3.6 — 8.4 — 9.8 — 16.8 End of period 117,127 $ 31.1 127,537 $ 50.5 117,127 $ 31.1 127,537 $ 50.5 (1) WTM performance share payments in 2015 for the 2012-2014 performance cycle ranged from 91% to 145.5% of target. WTM performance share payments in 2014 for the 2011-2013 performance cycle ranged from 88% to 131.5% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP.
Summary of performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan	The following table summarizes performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan at June 30, 2015 for each performance cycle: Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Performance cycle: 2015 – 2017 31,390 $ 3.4 2014 – 2016 41,937 5.1 2013 – 2015 46,803 23.4 Sub-total 120,130 31.9 Assumed forfeitures (3,003 ) (.8 ) Total at June 30, 2015 117,127 $ 31.1
Summary of restricted share activity	The following table summarizes the unrecognized compensation cost associated with the outstanding restricted share awards for the three and six months ended June 30, 2015 and 2014 : Three Months Ended June 30, Six Months Ended June 30, 2015 2014 2015 2014 Millions, except share amounts Restricted Unamortized Restricted Unamortized Restricted Shares Unamortized Issue Date Fair Value Restricted Shares Unamortized Issue Date Fair Value Non-vested, Beginning of period 71,125 $ 26.5 81,325 $ 24.8 83,314 $ 14.3 94,130 $ 17.0 Issued 750 .5 — — 23,640 15.7 20,400 11.8 Vested — — — — (35,079 ) — (33,205 ) — Forfeited — — — — — — — — Expense recognized — (3.8 ) — (3.9 ) — (6.8 ) — (7.9 ) End of period 71,875 $ 23.2 81,325 $ 20.9 71,875 $ 23.2 81,325 $ 20.9
Summary of performance share activity for OneBeacon performance shares granted under OneBeacon Incentive Plan	The following table summarizes performance share activity for the three and six months ended June 30, 2015 and 2014 for OneBeacon performance shares granted under the OneBeacon Incentive Plan: Three Months Ended June 30, Six Months Ended June 30, 2015 2014 2015 2014 Millions, except share amounts Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Target Performance Shares Outstanding Accrued Expense Beginning of period 462,147 $ 2.1 512,938 $ 3.6 517,470 $ 3.4 493,421 $ 4.0 Shares paid or expired (1) — — — — (181,290 ) (1.5 ) (142,138 ) (1.0 ) New grants — — — — 154,887 — 165,800 — Assumed forfeitures and cancellations (2) — — — — (28,920 ) — (4,145 ) — Expense recognized — .3 — .8 — .5 — 1.4 End of period 462,147 $ 2.4 512,938 $ 4.4 462,147 $ 2.4 512,938 $ 4.4 (1) OneBeacon performance share payments in 2015 for the 2012-2014 performance cycle were at 45.7% of target. OneBeacon performance share payments in 2014 for the 2011-2013 performance cycle were at 37.1% of target. (2) Amounts include changes in assumed forfeitures, as required under GAAP.
Summary of OneBeacon performance shares outstanding and accrued expense for OneBeacon performance shares awarded under the OneBeacon Incentive Plan	The following table summarizes OneBeacon performance shares outstanding awarded under the OneBeacon Incentive Plan at June 30, 2015 for each performance cycle: Millions, except share amounts Target Accrued Expense Performance cycle: 2015 – 2017 154,887 $ .3 2014 – 2016 151,810 .6 2013 – 2015 167,300 1.5 Sub-total 473,997 2.4 Assumed forfeitures (11,850 ) — Total at June 30, 2015 462,147 $ 2.4
Summary of OneBeacon restricted shares activity	The following table summarizes the unrecognized compensation cost associated with the outstanding OneBeacon restricted stock awards for the three and six months ended June 30, 2015 and 2014 : Three Months Ended June 30, Six Months Ended June 30, 2015 2014 2015 2014 Millions, except share amounts Restricted Unamortized Restricted Unamortized Restricted Unamortized Restricted Unamortized Non-vested, Beginning of period 390,950 $ 4.0 612,500 $ 5.7 612,500 $ 3.5 915,000 $ 6.5 Issued — — — — 75,950 1.1 — — Vested — — — — (296,000 ) — (300,000 ) — Forfeited — — — — (1,500 ) — (2,500 ) — Expense recognized — (.5 ) — (.8 ) — (1.1 ) — (1.6 ) End of period 390,950 $ 3.5 612,500 $ 4.9 390,950 $ 3.5 612,500 $ 4.9

Fair Value of Financial Instr39

Fair Value of Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 2015
Fair Value Disclosures [Abstract]
Summary of the fair value and carrying value of financial instruments	The following table summarizes the fair value and carrying value of financial instruments as of June 30, 2015 and December 31, 2014 : June 30, 2015 December 31, 2014 Millions Fair Value Carrying Value Fair Value Carrying Value 2012 OBH Senior Notes $ 280.4 $ 274.7 $ 286.0 $ 274.7 SIG Senior Notes 437.8 399.7 437.8 399.7 SIG Preference Shares 261.8 250.0 260.0 250.0 Tranzact Bank Facility 70.2 70.6 68.7 68.7

Discontinued Operations (Tables

Discontinued Operations (Tables)	6 Months Ended
	Jun. 30, 2015
Discontinued Operations and Disposal Groups [Abstract]
Schedule of Disposal Groups, Including Discontinued Operations, Income Statement, Balance Sheet And Additional Disclosures	The following table outlines the computation of earnings per share for discontinued operations for the three and six months ended June 30, 2015 and 2014 : Three Months Ended Six Months Ended June 30, June 30, 2015 2014 2015 2014 Basic and diluted earnings per share numerators (in millions): Net income attributable to White Mountains’s common shareholders $ (.3 ) $ 2.6 $ 7.6 $ 2.1 Allocation of income for participating unvested restricted common shares (1) — — (.1 ) — Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts (2) $ (.3 ) $ 2.6 $ 7.5 $ 2.1 Basic earnings per share denominators (in thousands): Total average common shares outstanding during the period 5,985.7 6,162.2 5,982.1 6,166.0 Average unvested restricted common shares (3) (71.4 ) (81.3 ) (65.2 ) (75.9 ) Basic earnings per share denominator 5,914.3 6,080.9 5,916.9 6,090.1 Diluted earnings per share denominator (in thousands): Total average common shares outstanding during the period 5,985.7 6,162.2 5,982.1 6,166.0 Average unvested restricted common shares (3) (71.4 ) (81.3 ) (65.2 ) (75.9 ) Average outstanding dilutive options to acquire common shares (4) — — — — Diluted earnings per share denominator 5,914.3 6,080.9 5,916.9 6,090.1 Basic and diluted earnings per share (in dollars): $ (.04 ) $ .43 $ 1.27 $ .34 (1) Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities. (2) Net earnings attributable to White Mountains’s common shareholders, net of restricted share amounts, is equal to undistributed earnings for the three and six months ended June 30, 2015 and 2014 . (3) Restricted common shares outstanding vest either in equal annual installments or upon a stated date. (See Note 15 - “Employee Share-Based Compensation Plans” ). (4) The diluted earnings per share denominator for the three and six months ended June 30, 2015 and 2014 does not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.

Subsequent Events (Tables)

Subsequent Events (Tables)	6 Months Ended
	Jun. 30, 2015
Subsequent Event [Line Items]
Industry Loss Warranty [Table Text Block]	The majority of the contracts expire in May or June of 2016. The following summarizes the ILW contracts in force: Scope Limit Industry Loss Trigger United States first event $75.0 million $40.0 billion United States first event $22.5 million $50.0 billion United States second event $45.0 million $15.0 billion Japan first event $25.0 million $12.5 billion

Summary of Significant Accoun42

Summary of Significant Accounting Policies (Basis of Presentation) (Details) - Equity Component [Domain] - Class of Stock [Domain] - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Jul. 27, 2015	Jul. 31, 2012
Basis of Presentation
Net investment income	$ 23.8	$ 29.6	$ 47.1	$ 54.3
Income Tax Expense (Benefit)	$ (2.7)	24.5	$ 22.3	55.4
OneBeacon Ltd.
Basis of Presentation
Cumulative Effect on Retained Earnings, Net of Tax					$ 0.9	$ 0.5
Ownership interest (as a percent)	75.30%		75.30%		75.30%
OneBeacon
Basis of Presentation
Proceeds from Sale of Buildings	$ 58
Noncontrolling equity percentage	24.70%		24.70%		24.70%
HG Global
Basis of Presentation
Surplus Notes								$ 503
Percentage of par value of policy reinsured	15.00%		15.00%
Noncontrolling equity percentage	3.10%		3.10%		3.10%
Star & Shield Insurance Exchange
Basis of Presentation
Surplus Notes	$ 20		$ 20
Noncontrolling equity percentage	100.00%		100.00%		100.00%
White Mountains Advisors L L C [Member]
Basis of Presentation
Investment Advisory, Management and Administrative Fees	$ 2	3	$ 4	6
Preferred Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	96.90%		96.90%
Noncontrolling equity percentage	96.90%		96.90%
Common Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	88.40%		88.40%
Noncontrolling equity percentage	88.40%		88.40%
Accounting Standards Update 2014-01 [Member] | Restatement Adjustment [Member] | OneBeacon Ltd.
Basis of Presentation
Net investment income		0.5		0.9
Income Tax Expense (Benefit)		$ 0.6		$ 1.2
Sale of Sirius [Member] | Sirius Group
Basis of Presentation
Percentage of common shareholder's equity							127.30%
Additional cash that will be paid as part of the purchase price							$ 10
Payments for (Proceeds from) Businesses and Interest in Affiliates			$ 2,200

Significant Transactions (Detai

Significant Transactions (Details) ₪ in Millions, $ in Millions	1 Months Ended		2 Months Ended				3 Months Ended					6 Months Ended
	Jan. 31, 2014USD ($)	Oct. 31, 2011USD ($)	Feb. 23, 2015ILS (₪)	Feb. 23, 2015USD ($)	Feb. 19, 2014ILS (₪)business	Feb. 19, 2014USD ($)business	Jun. 30, 2015ILS (₪)	Jun. 30, 2015USD ($)	Dec. 31, 2014USD ($)	Jun. 30, 2014USD ($)	Mar. 31, 2014USD ($)	Jun. 30, 2015USD ($)	Jun. 30, 2014USD ($)	Mar. 31, 2015USD ($)	Oct. 10, 2014USD ($)	Mar. 14, 2014USD ($)	Dec. 31, 2013USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates												$ 2.4	$ 32.2
Assets								$ 10,429.3	$ 10,455.7			10,429.3
Goodwill								169.2	168.9	$ 23.8	$ 23.8	169.2	23.8	$ 169.2			$ 0
Intangible Assets, Net (Excluding Goodwill)								186.4	197.5	59.8	62.4	186.4	59.8	$ 193.4			$ 20.7
Goodwill and intangible assets								355.6	366.4			355.6
Liabilities								5,970.8	5,917.3			5,970.8
Payments to Acquire Additional Interest in Subsidiaries												9.1	0
Net Income (Loss) Attributable to Noncontrolling Interest								$ 0.9		(4.6)		$ 8.2	(4)
Tranzact [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates		$ 177.7
Enterprise Value of Subsidiary															$ 281.2
Proceeds from Contributions from Affiliates									44.2
Ownership interest (as a percent)															63.20%
Assets															$ 332.8
Tangible Assets															41.4
Goodwill															145.1
Intangible Assets, Net (Excluding Goodwill)															146.3
Liabilities															$ 108.7
PPCI Global Ltd. [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share								50				50
Payments to Acquire Interest in Subsidiaries and Affiliates								$ 21
OneTitle [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Agreed Upon Commitment of Contributed Capital								13				$ 13
Star & Shield Insurance Exchange
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Purchase of Surplus Notes								20				20
Assets								13.4	13.5			13.4
Liabilities								27.8	25.9			27.8
QuoteLab
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business Combination, Contingent Consideration, Liability								$ 7.9	$ 7.9			$ 7.9
Payments to Acquire Businesses and Interest in Affiliates											$ 28.1
Percentage of gross profit																62.50%
Assets																$ 70.1
Goodwill																18.3
Goodwill and intangible assets																38.5
Liabilities																$ 10
WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Premiums Written, Percentage of Total					85.00%	85.00%
Number of lines of business | business					4	4
Goodwill						$ 5.5
Liabilities						$ 0.7
Star & Shield LLC [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates	$ 1.8
Common Class A | QuoteLab
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)																60.00%
Common Stock | Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)				56.20%				68.30%				68.30%
Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownership interest (as a percent)						54.00%		95.30%				95.30%
Non-controlling interest | Star & Shield Insurance Exchange
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net Income (Loss) Attributable to Noncontrolling Interest								$ 0.8		$ 5.2		$ 1.9	$ 9.7
Israel, New Shekels
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates | ₪							₪ 10
Israel, New Shekels | Convertible Preferred Stock [Member] | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Additional Interest in Subsidiaries | ₪							25
Israel, New Shekels | Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Additional Interest in Subsidiaries | ₪							₪ 35
Israel, New Shekels | Preferred Stock [Member] | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪					₪ 31.5
Israel, New Shekels | Common Stock | Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪			₪ 9.5
Israel, New Shekels | Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪					₪ 14.4
United States of America, Dollars
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates								2.6
United States of America, Dollars | Convertible Preferred Stock [Member] | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Additional Interest in Subsidiaries								6.6
United States of America, Dollars | Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Additional Interest in Subsidiaries								$ 9.1
United States of America, Dollars | Preferred Stock [Member] | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates						$ 8.5
United States of America, Dollars | Common Stock | Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates				$ 2.4
United States of America, Dollars | Common Stock | WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments to Acquire Businesses and Interest in Affiliates						4.1
Cashboard [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets				5.5
Goodwill				0.3
Intangible Assets, Net (Excluding Goodwill)				2.8
Liabilities				$ 1.2
WOBI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets						13.4
Intangible Assets, Net (Excluding Goodwill)						$ 2.9

Loss and Loss Adjustment Expe44

Loss and Loss Adjustment Expense Reserves (Loss and Loss Adjustment Expense) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Loss and Loss Adjustment Expense Reserve
Gross beginning balance	$ 3,035	$ 3,048.4	$ 3,159.8	$ 3,079.3
Less beginning reinsurance recoverable on unpaid losses	(436.9)	(425.2)	(483.9)	(428.1)
Net loss and LAE reserves	2,598.1	2,623.2	2,675.9	2,651.2
Loss and LAE reserves acquired				13.6
Loss and LAE reserves consolidated — SSIE	0	0	0
Loss and LAE incurred relating to current year losses	300	278.4	566.8	518.9
Net (favorable) and unfavorable loss reserve development	(11.7)	4.3	(13.1)	(6.9)
Total incurred losses and LAE	288.3	282.7	553.7	512
Foreign currency translation adjustment to net loss and LAE reserves	7.5	(0.5)	(14.9)	1.3
Current year losses	(72.9)	(65.8)	(113.5)	(100.8)
Prior year losses	(216.7)	(219.5)	(496.9)	(457.2)
Total loss and LAE payments	(289.6)	(285.3)	(610.4)	(558)
Net ending balance	2,604.3	2,620.1	2,604.3	2,620.1
Plus ending reinsurance recoverable on unpaid losses	427.5	433.2	427.5	433.2
Gross ending balance	$ 3,031.8	$ 3,053.3	$ 3,031.8	$ 3,053.3

Loss and Loss Adjustment Expe45

Loss and Loss Adjustment Expense Reserves Details (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	$ (11.7)	$ 4.3	$ (13.1)	$ (6.9)
Sirius Group
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	(11.4)	(6.3)	(10.9)	(16.1)
Star & Shield Insurance Exchange
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development	$ (0.3)	2.2	(0.4)	2.2
OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net (favorable) and unfavorable loss reserve development		$ 8.4	$ (1.8)	$ 7

Third Party Reinsurance (One Be

Third Party Reinsurance (One Beacon) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	$ 31.7	$ 23.6
Reinsurance recoverable on unpaid losses	427.5	483.9
OneBeacon
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	20.9	12.2
Reinsurance recoverable on unpaid losses	$ 147.1	$ 161.6
Crop insurance [Member] | OneBeacon
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured	52.00%
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage	98.00%
Percentage of Quota Share Reinsurance Agreement	30.00%
Crop-hail insurance [Member] | OneBeacon
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured	80.00%
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage	100.00%
OneBeacon
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	$ 130
Reinsurance Retention Policy, Amount Retained	$ 20
OneBeacon | Combination One
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured	95.00%
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation	$ 10
OneBeacon | Minimum | Property [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Amount Retained	3
OneBeacon | Minimum | Health Care [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Amount Retained	3
OneBeacon | Maximum | Property [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Amount Retained	5
OneBeacon | Maximum | Health Care [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Amount Retained	$ 5
OneBeacon | Combination Two
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured	100.00%
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation	$ 100

Third Party Reinsurance (Sirius

Third Party Reinsurance (Sirius Group) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	$ 31.7	$ 23.6
Reinsurance recoverable on unpaid losses	427.5	483.9
Sirius Group
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	10.8	11.4
Reinsurance recoverable on unpaid losses	$ 280.3	$ 322.2

Investment Securities (Net Inve

Investment Securities (Net Investment Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Investment income
Total investment income	$ 28.4	$ 34.4	$ 55.5	$ 63.9
Third-party investment expenses	(4.6)	(4.8)	(8.4)	(9.6)
Net investment income, pre-tax	23.8	29.6	47.1	54.3
Fixed maturity investments.
Investment income
Total investment income	23.4	25.6	46.5	48
Short-term investments
Investment income
Total investment income	0.7	0.6	1.3	1.2
Common Stock
Investment income
Total investment income	3.3	5.5	6.7	10.9
Convertible fixed maturity investments
Investment income
Total investment income	0.1	0.8	0.3	1.2
Other long-term investments
Investment income
Total investment income	0.9	1.9	0.8	2.6
Funds Held under Reinsurance Agreements [Member]
Investment income
Total investment income	$ 0	$ 0	$ (0.1)	$ 0

Investment Securities (Net Real

Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	$ 85.7	$ 27.2	$ 111.5	$ 52.5
Trading Securities, Realized Gain (Loss), Total	112.9	30.5	165.7	52.2
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(174)	83.4	(123.4)	125.5
Net Realized and Unrealized Gain (Loss) on Trading Securities	(61.1)	113.9	42.3	177.7
Income tax benefit (expense) attributable to net realized and unrealized investment gains (losses)	16.5	(23)	(9.8)	(38.7)
Realized and unrealized gain	(44.6)	90.9	32.5	139
Foreign Currency Transaction Gain (Loss), Realized	27.2	3.3	54.2	(0.3)
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(24.6)	(5.3)	(31.1)	(9.9)
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	(6.4)	(1)	(13.1)	(0.1)
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(31)	(6.3)	(44.2)	(10)
Trading Securities Realized Holding Gain (Loss) on Investments after Tax	61.1	21.9	80.4	42.6
Trading Securities, Realized Foreign Currency Gain (Loss) Net of Tax	20.8	2.3	41.1	(0.4)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	81.9	24.2	121.5	42.2
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	2.7	5.7	12.4	10.1
Trading Securities, Realized Gain (Loss), Total	17	8.7	47.9	9.6
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(73.7)	53.1	(21.4)	84.7
Foreign Currency Transaction Gain (Loss), Realized	14.3	3	35.5	(0.5)
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0	0	0	0
Trading Securities, Realized Gain (Loss), Total	5.7	0	9.3	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0	0	0	0
Foreign Currency Transaction Gain (Loss), Realized	5.7	0	9.3	0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	65.9	19.4	90.3	38.3
Trading Securities, Realized Gain (Loss), Total	73.1	19.4	98.6	38.2
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(72.1)	22.4	(75.6)	26.9
Foreign Currency Transaction Gain (Loss), Realized	7.2	0	8.3	(0.1)
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0.6	1.5	(3.6)	3.9
Trading Securities, Realized Gain (Loss), Total	0.6	1.5	(3.6)	3.9
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(0.1)	(2.1)	0.7	(1.4)
Foreign Currency Transaction Gain (Loss), Realized	0	0	0	0
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	16.9	0.6	12.8	0.3
Trading Securities, Realized Gain (Loss), Total	16.9	0.9	13.9	0.6
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(28.1)	10	(27.1)	15.3
Foreign Currency Transaction Gain (Loss), Realized	0	0.3	1.1	0.3
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(0.4)	0	(0.4)	(0.1)
Trading Securities, Realized Gain (Loss), Total	(0.4)	0	(0.4)	(0.1)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0	0	0	0
Foreign Currency Transaction Gain (Loss), Realized	$ 0	$ 0	$ 0	$ 0

Investment Securities (Net Re50

Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	$ 85.7	$ 27.2	$ 111.5	$ 52.5
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(24.6)	(5.3)	(31.1)	(9.9)
Trading securities realized holding gain (loss) on investments after tax	61.1	21.9	80.4	42.6
Foreign Currency Transaction Gain (Loss), Realized	27.2	3.3	54.2	(0.3)
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	(6.4)	(1)	(13.1)	(0.1)
Trading securities, realized foreign currency gain (loss) net of tax	20.8	2.3	41.1	(0.4)
Trading Securities, Realized Gain (Loss), Total	112.9	30.5	165.7	52.2
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(31)	(6.3)	(44.2)	(10)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	81.9	24.2	121.5	42.2
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	2.7	5.7	12.4	10.1
Foreign Currency Transaction Gain (Loss), Realized	14.3	3	35.5	(0.5)
Trading Securities, Realized Gain (Loss), Total	17	8.7	47.9	9.6
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0	0	0	0
Foreign Currency Transaction Gain (Loss), Realized	5.7	0	9.3	0
Trading Securities, Realized Gain (Loss), Total	5.7	0	9.3	0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	65.9	19.4	90.3	38.3
Foreign Currency Transaction Gain (Loss), Realized	7.2	0	8.3	(0.1)
Trading Securities, Realized Gain (Loss), Total	73.1	19.4	98.6	38.2
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0.6	1.5	(3.6)	3.9
Foreign Currency Transaction Gain (Loss), Realized	0	0	0	0
Trading Securities, Realized Gain (Loss), Total	0.6	1.5	(3.6)	3.9
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	16.9	0.6	12.8	0.3
Foreign Currency Transaction Gain (Loss), Realized	0	0.3	1.1	0.3
Trading Securities, Realized Gain (Loss), Total	16.9	0.9	13.9	0.6
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(0.4)	0	(0.4)	(0.1)
Foreign Currency Transaction Gain (Loss), Realized	0	0	0	0
Trading Securities, Realized Gain (Loss), Total	$ (0.4)	$ 0	$ (0.4)	$ (0.1)

Investment Securities (Net Unre

Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (127.7)	$ 52.2	$ (118.2)	$ 81.2
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	36	(9.8)	33.2	(18.9)
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	(91.7)	42.4	(85)	62.3
Foreign Currency Transaction Gain (Loss), Unrealized	(46.3)	31.2	(5.2)	44.3
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	11.5	(6.9)	1.2	(9.8)
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(34.8)	24.3	(4)	34.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(174)	83.4	(123.4)	125.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	47.5	(16.7)	34.4	(28.7)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(126.5)	66.7	(89)	96.8
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(34.7)	24.9	(22.6)	44.3
Foreign Currency Transaction Gain (Loss), Unrealized	(39)	28.2	1.2	40.4
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(73.7)	53.1	(21.4)	84.7
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0	0	0	0
Foreign Currency Transaction Gain (Loss), Unrealized	0	0	0	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0	0	0	0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(63.9)	21	(68.1)	25
Foreign Currency Transaction Gain (Loss), Unrealized	(8.2)	1.4	(7.5)	1.9
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(72.1)	22.4	(75.6)	26.9
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0	(2.4)	0.7	(1.7)
Foreign Currency Transaction Gain (Loss), Unrealized	(0.1)	0.3	0	0.3
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(0.1)	(2.1)	0.7	(1.4)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(29.1)	8.7	(28.2)	13.6
Foreign Currency Transaction Gain (Loss), Unrealized	1	1.3	1.1	1.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(28.1)	10	(27.1)	15.3
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0	0	0	0
Foreign Currency Transaction Gain (Loss), Unrealized	0	0	0	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	$ 0	$ 0	$ 0	$ 0

Investment Securities (Investme

Investment Securities (Investment gains (losses) for Level 3) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (127.7)	$ 52.2	$ (118.2)	$ 81.2
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(34.7)	24.9	(22.6)	44.3
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(63.9)	21	(68.1)	25
Convertible Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0	(2.4)	0.7	(1.7)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(29.1)	8.7	(28.2)	13.6
Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(21.3)	14.3	(26.5)	21.4
Level 3 Inputs | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.3	0.2	(0.4)	0.4
Level 3 Inputs | Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	4.9	2	3.1	2.8
Level 3 Inputs | Convertible Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.7	3.2	0.8	3.2
Level 3 Inputs | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (27.2)	$ 8.9	$ (30)	$ 15

Investment Securities (Invest53

Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,874.5	$ 4,784.3
U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	183.9	184.7
Trading Securities, Unrealized Holding Gain	0.3	0.1
Trading Securities, Unrealized Holding Loss	(0.2)	(0.3)
Net foreign currency gains (losses)	(0.4)	3.6
Fixed maturity investments, at fair value	183.6	188.1
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	2,232.7	2,221.3
Trading Securities, Unrealized Holding Gain	27.7	45.2
Trading Securities, Unrealized Holding Loss	(7)	(5.1)
Net foreign currency gains (losses)	57.7	49.8
Fixed maturity investments, at fair value	2,311.1	2,311.2
Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	118	82
Trading Securities, Unrealized Holding Gain	0.8	1.4
Trading Securities, Unrealized Holding Loss	(1.1)	(0.2)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	117.7	83.2
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	2,057.4	1,811.1
Trading Securities, Unrealized Holding Gain	9.2	7.6
Trading Securities, Unrealized Holding Loss	(4.7)	(3.5)
Net foreign currency gains (losses)	15.5	25.7
Fixed maturity investments, at fair value	2,077.4	1,840.9
Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	98.9	274.6
Trading Securities, Unrealized Holding Gain	0.3	4.2
Trading Securities, Unrealized Holding Loss	(1.1)	(1)
Net foreign currency gains (losses)	0.6	(2.7)
Fixed maturity investments, at fair value	98.7	275.1
Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	79.5	79.6
Trading Securities, Unrealized Holding Gain	6.3	6.1
Trading Securities, Unrealized Holding Loss	0	0
Net foreign currency gains (losses)	0.2	0.1
Fixed maturity investments, at fair value	86	85.8
Fixed Income Investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	4,770.4	4,653.3
Trading Securities, Unrealized Holding Gain	44.6	64.6
Trading Securities, Unrealized Holding Loss	(14.1)	(10.1)
Net foreign currency gains (losses)	73.6	76.5
Total fixed maturity investments including assets held for sale	$ 4,874.5	$ 4,784.3

Investment Securities (Invest54

Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,874.5	$ 4,784.3
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	656.7	633.6
Trading Securities, Unrealized Holding Gain	69.1	175.1
Trading Securities, Unrealized Holding Loss	(12.2)	(5.2)
Net foreign currency gains (losses)	0.2	(1.9)
Fixed maturity investments, at fair value	713.8	801.6
Convertible Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	3.1	19.1
Trading Securities, Unrealized Holding Gain	1.4	0.9
Trading Securities, Unrealized Holding Loss	0	(0.2)
Net foreign currency gains (losses)	0	0.7
Fixed maturity investments, at fair value	4.5	20.5
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	346	343.3
Trading Securities, Unrealized Holding Gain	54	73
Trading Securities, Unrealized Holding Loss	(11.9)	(10.9)
Net foreign currency gains (losses)	2	1.6
Fixed maturity investments, at fair value	$ 390.1	$ 407

Investment Securities (Other lo

Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Payments to Acquire Other Investments		$ 33.3	$ 17.7
Other Investments	$ 390.1	390.1		$ 407
Partnership investments [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	3.9	3.9		5.2
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	210.8	210.8		242.9
Surplus Note
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	66.3	66.3		65.1
Limited Liability Companies and Private Equity Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	89.9	89.9		69.7
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	15.8	15.8	$ 18	16.8	$ 19.1
Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	3.4	3.4		7.3
PassportCard [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Payments to Acquire Other Investments	21
Hedge Funds, Equity Long (Short) [Member] | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unfunded Commitments	0	0		0
Hedge Funds Equity R E I T [Member] | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	20.5	20.5		20.3
Unfunded Commitments	$ 0	$ 0		$ 0

Investment Securities (Hedge Fu

Investment Securities (Hedge Funds and Private Equity Funds) (Details) $ in Millions	Jun. 30, 2015USD ($)fund	Dec. 31, 2014USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$ 390.1	$ 407
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments | fund	14
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of investments | fund	29
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Largest single fund investment	$ 25

Investment Securities (Fair Val

Investment Securities (Fair Value of Hedge Funds and Private Equity Funds) (Details) - Advance Notice Period [Domain] - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 77.7
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	133.1
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	210.8	$ 242.9
Unfunded Commitments	94.8	81.7
Private equity funds | Private Equity Multi Sector
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	24.5	24.2
Unfunded Commitments	4.4	5.3
Private equity funds | Private Equity Energy Infrastructure and Services
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	45.7	59.6
Unfunded Commitments	9.9	11
Private equity funds | Real Estate Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	1.8	3.6
Unfunded Commitments	0.1	3.3
Private equity funds | Private Equity Secondaries
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	7.4	8.5
Unfunded Commitments	3.2	3.1
Private equity funds | Private Equity Funds Multi Sector Europe
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.4	1.5
Unfunded Commitments	0	2.3
Private equity funds | Private Equity Funds Manufacturing and Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	25.9	23.2
Unfunded Commitments	2.9	7.3
Private equity funds | Private Equity Healthcare
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	6.3	6.1
Unfunded Commitments	0.8	2.8
Private equity funds | Private Equity Insurance
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	2.1	2.1
Unfunded Commitments	41.3	41.2
Private equity funds | Aerospace Defense Government
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	18.7	20.7
Unfunded Commitments	32.2	5.1
Private equity funds | Venture Capital Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0.3	1.4
Unfunded Commitments	0	0.3
Private equity funds | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	133.1	150.9
Unfunded Commitments	94.8	81.7
Hedge funds | Hedge Fund, Long/ Short Equity Banks and Financial [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	15.9	29.9
Hedge funds | Hedge Funds Equity R E I T [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	20.5	20.3
Unfunded Commitments	0	0
Hedge funds | Hedge Funds, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	22.1	20.4
Hedge funds | Hedge Funds Credit and Distressed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	19.2	21.4
Unfunded Commitments	0	0
Hedge funds | Hedge funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	77.7	92
Unfunded Commitments	$ 0	$ 0

Investment Securities (Restrict

Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) - Hedge funds $ in Millions	Jun. 30, 2015USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Distributions from Inactive Hedge Funds	$ 2
Other long-term investments	77.7
Redemption requests	10.2
30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	23
Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	41.7
Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	4.9
120+ days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	8.1
Monthly
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	5.2
Monthly | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	5.2
Monthly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Monthly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Monthly | 120+ days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Quarterly
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	45
Quarterly | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	17.8
Quarterly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	19.2
Quarterly | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Quarterly | 120+ days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	8
Semi-annual
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	22.5
Semi-annual | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Semi-annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	22.5
Semi-annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Semi-annual | 120+ days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Annual
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	5
Annual | 30-59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period60 To89 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	0
Annual | Fair Value Investments Entities that Calculate Net Asset Value Per Share Investment Redemption Notice Period90 To119 Days [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	4.9
Annual | 120+ days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 0.1

Investment Securities (Invest59

Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net Income (Loss) Attributable to Noncontrolling Interest	$ (0.9)	$ 4.6	$ (8.2)	$ 4
Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	133.1		133.1
1-3 years | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	27		27
3-5 years | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	29.5		29.5
5-10 years | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	72.5		72.5
Greater than 10 years | Private equity funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other long-term investments	$ 4.1		$ 4.1
Prospector Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity Method Investment, Aggregate Cost					$ 64.9
Variable Interest Entity, Consolidated, Carrying Amount, Assets					135.8
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities					39.8
Net Income (Loss) Attributable to Noncontrolling Interest					$ 31.1

Investment Securities (Fair V60

Investment Securities (Fair Value Measurements) (Details) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Maximum percentage of price difference provided by pricing services	5.00%
Maximum price difference provided by pricing services	$ 1
Fair value, investment-related liabilities		$ 38

Investment Securities (Fair V61

Investment Securities (Fair Value Measurement by Level) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,963.4	$ 5,991.4	$ 6,777	$ 6,766.5
Other Investments	390.1	407
Short-term investments, at amortized cost (which approximates fair value)	840.5	871.7
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	729.9	843.3	1,491.6	1,376.7
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,746.7	4,638	4,774	4,982.2
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,311.1	2,311.2
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,077.4	1,840.9
Mortgage-backed and asset-backed securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,070	1,840.9
Mortgage-backed and asset-backed securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	7.4	0
Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	78.6	76.7	158.4	93
Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	33.1	40.2	53.8	46.1
Convertible fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4.5	8.2	10.8	6.1
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	370.6	385	288.4	262.4
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	6,803.9	6,863.1
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,568.2	1,712.1
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,748.9	4,640.9
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	486.8	510.1
Fair value measured on a recurring basis | U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	183.6	188.1
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	131.5	134.1
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	52.1	54
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,311.1	2,311.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,311.1	2,305.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	5.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	544	575.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	544	575.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	281.3	225.4
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	281.3	219.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	5.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	405	431.4
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	405	431.4
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	237.2	214.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	237.2	214.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	306.2	284.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	306.2	284.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Health Care [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	194.2	177.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	194.2	177.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments		0
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	90.3	118
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	90.3	118
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	149	173.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	149	173.6
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	98.4	103
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	98.4	103
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Other:
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	5.5	8.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	5.5	8.1
Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	117.7	83.2
Fair value measured on a recurring basis | Municipal obligations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Municipal obligations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	117.7	83.2
Fair value measured on a recurring basis | Municipal obligations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,077.4	1,840.9
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,070	1,840.9
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	7.4	0
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	98.7	275.1
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	18.8	21.3
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	79.9	253.8
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	86	85.8
Fair value measured on a recurring basis | Preferred Stock | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Preferred Stock | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	14.8	14.7
Fair value measured on a recurring basis | Preferred Stock | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	71.2	71.1
Fair value measured on a recurring basis | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,874.5	4,784.3
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	150.3	155.4
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,645.6	4,552.2
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	78.6	76.7
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	840.5	871.7
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	838.3	868.8
Short-term investments, at amortized cost (which approximates fair value)	840.5	871.7	564	635.9
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2.2	2.9
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Consumer
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	51.6	161.2
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	51.6	161.1
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0.1
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Industrial
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	32.3	67.5
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	32.3	67.5
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Financials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	93.7	237.5
Fair value measured on a recurring basis | Common equity securities | Financials | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	60.6	197.3
Fair value measured on a recurring basis | Common equity securities | Financials | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Financials | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	33.1	40.2
Fair value measured on a recurring basis | Common equity securities | Communications [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	23.8	32.9
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	23.8	32.9
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Communications [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Health Care [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	25.8	101.9
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	25.8	101.9
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Health Care [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Energy
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2.5	32.6
Fair value measured on a recurring basis | Common equity securities | Energy | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2.5	32.6
Fair value measured on a recurring basis | Common equity securities | Energy | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Energy | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Technology
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	38.2	54.3
Fair value measured on a recurring basis | Common equity securities | Technology | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	38.2	54.3
Fair value measured on a recurring basis | Common equity securities | Technology | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Technology | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Utilities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1.1	9.5
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1.1	9.4
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0.1
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Materials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3.6	21.2
Fair value measured on a recurring basis | Common equity securities | Materials | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	3.6	21.2
Fair value measured on a recurring basis | Common equity securities | Materials | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Materials | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Other:
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	79.7	83
Fair value measured on a recurring basis | Common equity securities | Other: | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	9.7
Fair value measured on a recurring basis | Common equity securities | Other: | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	79.7	73.3
Fair value measured on a recurring basis | Common equity securities | Other: | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	361.5
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member] | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	340.1
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member] | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	21.4
Fair value measured on a recurring basis | Common equity securities | Exchange Traded Funds [Member] | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Fair value measured on a recurring basis | Common equity securities | Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	713.8	801.6
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	579.6	687.9
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	101.1	73.5
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	33.1	40.2
Fair value measured on a recurring basis | Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4.5	20.5
Fair value measured on a recurring basis | Convertible fixed maturity investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Convertible fixed maturity investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	12.3
Fair value measured on a recurring basis | Convertible fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4.5	8.2
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	370.6	385
Carrying value of investment accounted for using the equity method	3.9
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	370.6	385
Carrying value of investment accounted for using the equity method		5.2	5.6	6.8
Fair value measured on a recurring basis | Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	(0.2)
Segment, Discontinued Operations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments reclassified to assets held for sale			203.8	236.3
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	15.8	16.8	$ 18	$ 19.1
Affordable Housing Development Fund [Member] | Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$ 15.8	$ 16.8

Investment Securities (Debt sec

Investment Securities (Debt securities issued by corporation) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,963.4	$ 5,991.4	$ 6,777	$ 6,766.5
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,311.1	2,311.2
AAA | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
AA | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	200.5	236.9
A | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	887.6	957.8
BBB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,213.5	1,105.9
BB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	6.4	0
Other | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 3.1	$ 10.6

Investment Securities (Mortgage

Investment Securities (Mortgage-backed, Asset-backed Securities) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,963.4	$ 5,991.4	$ 6,777	$ 6,766.5
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 301	$ 236.9

Investment Securities (Mortga64

Investment Securities (Mortgage-backed Asset Securities Table) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,963.4	$ 5,991.4	$ 6,777	$ 6,766.5
Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	204.2
Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	355.4	411.2
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	38.9	36.6
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	34.8	49.6
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	429.1	497.4
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	204.5	131.2
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	301	236.9
Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	505.5	368.1
Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	934.6	865.5
Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	518.6	522.2
Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	430	289.4
Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	194.2	163.8
Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,142.8	975.4
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,077.4	1,840.9
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4,746.7	4,638	$ 4,774	$ 4,982.2
Level 2 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	355.4	411.2
Level 2 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	38.9	36.6
Level 2 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	34.8	49.6
Level 2 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	429.1	497.4
Level 2 Inputs | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	204.5	131.2
Level 2 Inputs | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	301	236.9
Level 2 Inputs | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	505.5	368.1
Level 2 Inputs | Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	934.6	865.5
Level 2 Inputs | Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	518.6	522.2
Level 2 Inputs | Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	430	289.4
Level 2 Inputs | Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	186.8	163.8
Level 2 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,135.4	975.4
Level 2 Inputs | Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,070	1,840.9
Level 3 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Collateralized Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Collateralized Credit Card Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Collateralized Auto Loans
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Level 3 Inputs | Collateralized Securities, Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	7.4	0
Level 3 Inputs | Asset-backed Securities, Securitized Loans and Receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	7.4	0
Level 3 Inputs | Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 7.4	$ 0
Maximum | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Loans and Leases Receivable, Ratio of Nonperforming Loans to All Loans	1.00%

Investment Securities (Non-agen

Investment Securities (Non-agency Mortgage-backed Securities) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,963.4	$ 5,991.4	$ 6,777	$ 6,766.5
Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	204.2
Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	96.8
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	204.5	131.2
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	301	236.9
Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	505.5	$ 368.1
Securities Issued in 2004 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	33
Securities Issued in 2004 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued in 2004 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	33
Securities Issued in 2005 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	12.3
Securities Issued in 2005 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued in 2005 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	12.3
Securities Issued In 2006 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.7
Securities Issued In 2006 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	8.3
Securities Issued In 2006 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	17
Securities Issued In 2007 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2007 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2007 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2008 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	14
Securities Issued In 2008 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2008 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	14
Securities Issued In 2009 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2009 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2009 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2010 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	47.4
Securities Issued In 2010 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	10.7
Securities Issued In 2010 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	58.1
Securities Issued In 2011 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	18.4
Securities Issued In 2011 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued In 2011 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	18.4
Securities Issued in 2012 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	4.1
Securities Issued in 2012 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	25.8
Securities Issued in 2012 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	29.9
Securities Issued in 2013 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	25.8
Securities Issued in 2013 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	58.6
Securities Issued in 2013 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	84.4
Securities Issued in 2014 | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	40.8
Securities Issued in 2014 | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	121.4
Securities Issued in 2014 | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	162.2
Securities Issued in 2015 [Member] | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Securities Issued in 2015 [Member] | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	76.2
Securities Issued in 2015 [Member] | Mortgage-backed Securities, Issued by Private Enterprises
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	76.2
Subordinate | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	101.3
Subordinate | Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	96.8
Subordinate | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Subordinate | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	198.1
Super Senior | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	18.4
Super Senior | Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Super Senior | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	69.1
Super Senior | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	18.4
Senior | Commercial Mortgage Backed Securities Fixed Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	84.5
Senior | Commercial Mortgage Backed Securities Floating Rate [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Senior | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	135.4
Senior | Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 84.5
Non-Agency Commercial Mortgage Backed Securities [Member] [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Basis points	2.50%

Investment Securities (Non-ag66

Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,963.4	$ 5,991.4	$ 6,777	$ 6,766.5
Non-Agency Residential Mortgage [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	73.1
Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	204.5	$ 131.2
Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	69.1
Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	135.4
Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Prime | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	202.5
Prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	69.1
Prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	133.4
Prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Non-prime | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2
Non-prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Non-prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2
Non-prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Sub-prime | Residential Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Sub-prime | Residential Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Sub-prime | Residential Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0
Sub-prime | Residential Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 0

Investment Securities (Non-ag67

Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,963.4	$ 5,991.4	$ 6,777	$ 6,766.5
Commercial Mortgage Backed Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	301	$ 236.9
Commercial Mortgage Backed Securities | Super Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	18.4
Commercial Mortgage Backed Securities | Senior
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	84.5
Commercial Mortgage Backed Securities | Subordinate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 198.1
Non-Agency Commercial Mortgage Backed Securities [Member] [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Basis points	2.50%

Rollforward of Fair Value Measu

Rollforward of Fair Value Measurements by Level (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2015USD ($)	Jun. 30, 2014USD ($)	Jun. 30, 2015USD ($)	Jun. 30, 2014USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			$ 5,991,400,000	$ 6,766,500,000
Total realized and unrealized gains (losses)			34,300,000	180,700,000
Foreign currency gains (losses) through OCI			(70,200,000)	(59,400,000)
Amortization/Accretion			(21,200,000)	(22,400,000)
Purchases			3,057,400,000	2,685,500,000
Sales			(2,960,800,000)	(2,778,500,000)
Net change in investments related to (sales) purchases of consolidated affiliates				4,600,000
Fair Value, Assets Measured on Recurring Basis, Other			(67,500,000)
Transfers in			73,900,000	28,100,000
Transfers out			(73,900,000)	(28,100,000)
Balance at June 30, 2015	$ 5,963,400,000	$ 6,777,000,000	5,963,400,000	6,777,000,000
Other Investments	390,100,000		390,100,000		$ 407,000,000
Realized and unrealized gain	(44,600,000)	90,900,000	32,500,000	139,000,000
Net realized and unrealized investment (losses) gains	(61,100,000)	113,900,000	42,300,000	177,700,000
Short-term investments, at amortized cost (which approximates fair value)	840,500,000		$ 840,500,000		871,700,000
Maximum Percentage Differences Between Prices Provided by Pricing Services Considered Outliers			(5.00%)
Maximum price difference provided by pricing services			$ 1,000,000
Price Difference Between the Investment Management Pricing and Third Party Pricing	1,000,000		1,000,000
Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			843,300,000	1,376,700,000
Total realized and unrealized gains (losses)			13,100,000	67,300,000
Foreign currency gains (losses) through OCI			(5,400,000)	(6,800,000)
Amortization/Accretion			(100,000)	(300,000)
Purchases			945,900,000	953,500,000
Sales			(1,017,800,000)	(896,100,000)
Net change in investments related to (sales) purchases of consolidated affiliates				(2,700,000)
Fair Value, Assets Measured on Recurring Basis, Other			(49,100,000)
Transfers in			0	0
Transfers out			0	0
Balance at June 30, 2015	729,900,000	1,491,600,000	729,900,000	1,491,600,000
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			4,638,000,000	4,982,200,000
Total realized and unrealized gains (losses)			31,300,000	89,800,000
Foreign currency gains (losses) through OCI			(62,300,000)	(50,800,000)
Amortization/Accretion			(21,100,000)	(22,000,000)
Purchases			2,002,400,000	1,632,000,000
Sales			(1,915,500,000)	(1,875,600,000)
Net change in investments related to (sales) purchases of consolidated affiliates				7,300,000
Fair Value, Assets Measured on Recurring Basis, Other			0
Transfers in			73,900,000	19,600,000
Transfers out			0	(8,500,000)
Balance at June 30, 2015	4,746,700,000	4,774,000,000	4,746,700,000	4,774,000,000
Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			76,700,000	93,000,000
Total realized and unrealized gains (losses)			0	900,000
Foreign currency gains (losses) through OCI			(200,000)	(400,000)
Amortization/Accretion			0	(100,000)
Purchases			76,000,000	76,100,000
Sales			0	0
Net change in investments related to (sales) purchases of consolidated affiliates				0
Fair Value, Assets Measured on Recurring Basis, Other			0
Transfers in			0	8,500,000
Transfers out			(73,900,000)	(19,600,000)
Balance at June 30, 2015	$ 78,600,000	158,400,000	$ 78,600,000	158,400,000
Number of Investments	5		5
Common equity securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			$ 40,200,000	46,100,000
Total realized and unrealized gains (losses)			6,900,000	2,700,000
Foreign currency gains (losses) through OCI			0	0
Amortization/Accretion			0	0
Purchases			0	5,000,000
Sales			(9,500,000)	0
Net change in investments related to (sales) purchases of consolidated affiliates				0
Fair Value, Assets Measured on Recurring Basis, Other			(4,500,000)
Transfers in			0	0
Transfers out			0	0
Balance at June 30, 2015	$ 33,100,000	53,800,000	33,100,000	53,800,000
Convertible Debt Securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			8,200,000	6,100,000
Total realized and unrealized gains (losses)			(3,700,000)	3,200,000
Foreign currency gains (losses) through OCI			0	0
Amortization/Accretion			0	0
Purchases			0	1,500,000
Sales			0	0
Net change in investments related to (sales) purchases of consolidated affiliates				0
Fair Value, Assets Measured on Recurring Basis, Other			0
Transfers in			0	0
Transfers out			0	0
Balance at June 30, 2015	4,500,000	10,800,000	4,500,000	10,800,000
Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			385,000,000	262,400,000
Total realized and unrealized gains (losses)			(13,300,000)	16,800,000
Foreign currency gains (losses) through OCI			(2,300,000)	(1,400,000)
Amortization/Accretion			0	0
Purchases			33,100,000	17,400,000
Sales			(18,000,000)	(6,800,000)
Net change in investments related to (sales) purchases of consolidated affiliates				0
Fair Value, Assets Measured on Recurring Basis, Other			(13,900,000)
Transfers in			0	0
Transfers out			0	0
Balance at June 30, 2015	370,600,000	288,400,000	370,600,000	288,400,000
Fair value measured on a recurring basis
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			6,863,100,000
Balance at June 30, 2015	6,803,900,000		6,803,900,000
Fair value measured on a recurring basis | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			1,712,100,000
Balance at June 30, 2015	1,568,200,000		1,568,200,000
Fair value measured on a recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			4,640,900,000
Balance at June 30, 2015	4,748,900,000		4,748,900,000
Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			510,100,000
Balance at June 30, 2015	486,800,000		486,800,000
Fair value measured on a recurring basis | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			4,784,300,000
Balance at June 30, 2015	4,874,500,000		4,874,500,000
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			155,400,000
Balance at June 30, 2015	150,300,000		150,300,000
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			4,552,200,000
Balance at June 30, 2015	4,645,600,000		4,645,600,000
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			76,700,000
Balance at June 30, 2015	78,600,000		78,600,000
Fair value measured on a recurring basis | Convertible Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			20,500,000
Balance at June 30, 2015	4,500,000		4,500,000
Fair value measured on a recurring basis | Convertible Debt Securities | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			0
Balance at June 30, 2015	0		0
Fair value measured on a recurring basis | Convertible Debt Securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			12,300,000
Balance at June 30, 2015	0		0
Fair value measured on a recurring basis | Convertible Debt Securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			8,200,000
Balance at June 30, 2015	4,500,000		4,500,000
Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			385,000,000
Balance at June 30, 2015	370,600,000		370,600,000
Carrying value of investment accounted for using the equity method	3,900,000		3,900,000
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			0
Balance at June 30, 2015	0		0
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			0
Balance at June 30, 2015	0		0
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			385,000,000
Balance at June 30, 2015	370,600,000		370,600,000
Carrying value of investment accounted for using the equity method		5,600,000		5,600,000	5,200,000	$ 6,800,000
Fair value measured on a recurring basis | Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at June 30, 2015	(200,000)		(200,000)
Fair value measured on a recurring basis | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			871,700,000
Balance at June 30, 2015	840,500,000		840,500,000
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			868,800,000
Balance at June 30, 2015	838,300,000		838,300,000
Short-term investments, at amortized cost (which approximates fair value)	840,500,000	564,000,000	840,500,000	564,000,000	871,700,000	635,900,000
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			2,900,000
Balance at June 30, 2015	2,200,000		2,200,000
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015			0
Balance at June 30, 2015	0		0
Segment, Discontinued Operations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments reclassified to assets held for sale		203,800,000		203,800,000		236,300,000
Prospector Offshore Fund Ltd [Member] | Fair value measured on a recurring basis | Investment related liabilities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment (losses) gains			900,000	1,800,000
Forward Contracts | Fair value measured on a recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Balance at January 1, 2015				(100,000)
Balance at June 30, 2015	(200,000)	100,000	(200,000)	100,000
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	15,800,000	$ 18,000,000	15,800,000	$ 18,000,000	16,800,000	$ 19,100,000
Affordable Housing Development Fund [Member] | Fair value measured on a recurring basis | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$ 15,800,000		15,800,000		$ 16,800,000
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized and unrealized investment (losses) gains			$ 9.3

Investment Securities (Signific

Investment Securities (Significant Unobservable Inputs) (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2015USD ($)inputInvestment	Dec. 31, 2014USD ($)
Quantitative information for Level 3 Fair Value Measurements Assets
Number of Investments with Significant Unobseravble Inputs | Investment	1
Conditional Prepayment Rate	12.00%
Discount Yield [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value Inputs, Discount Rate
NR | Preferred Stock | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 71,200,000	$ 71,100,000
NR | Preferred Stock | Discount Yield [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value Inputs, Discount Rate	7.10%	7.10%
NR | Private equity funds | Multiple of GAAP book value [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 33,100,000	$ 40,200,000
Book Value Multiple to Calculate Fair Value	120.00%	110.00%
NR | Private equity funds | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 20,100,000	$ 20,100,000
NR | Private equity funds | Share Price of Most Recent Transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Share Price of Most Recent Transaction	$ 1.06	1.06
NR | Convertible Preferred Stock [Member] | Multiple of EBITA [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Unobservable Inputs | input	0
Assets, Fair Value Disclosure	$ 4,500,000	3,800,000
Share Price of Most Recent Transaction	6	6
NR | Convertible Preferred Stock [Member] | Recent transaction [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		4,500,000
Share Price of Most Recent Transaction		0.71
NR | Seller Priority Surplus Note [Member] | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 45,400,000	$ 44,000,000
Fair Value Inputs, Discount Rate	10.20%	9.30%
Timing of interest payments	5 years	5 years
Timing of principal payments	10 years	10 years
NR | Pari Passu Surplus Note [Member] | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 20,900,000	$ 21,100,000
Fair Value Inputs, Discount Rate	15.50%	13.50%
Timing of interest payments	5 years	5 years
Timing of principal payments	15 years	10 years
Standard & Poor's, A- Rating | Non-Agency Commercial Mortgage Backed Securities [Member] [Member] | Broker Pricing [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	$ 7,400,000
durchblicker.at [Member] | NR | Private equity funds | Recent Transaction Two [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	9,500,000	$ 10,400,000
Share Price of Most Recent Transaction	290.96	290.96
BE Reinsurance [Member] | NR | Private equity funds | Recent Transaction Three [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure	16,100,000	15,800,000
Share Price of Most Recent Transaction	$ 0.13	0.13
Centrus [Member] | NR | Debt securities issued by corporations | Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, Fair Value Disclosure		$ 5,600,000
Fair Value Inputs, Discount Rate		10.00%
Minimum | discount spread [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Basis points	0.45%
Minimum | Default Rate [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Constant Default Rate	2.50%
Maximum | discount spread [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Basis points	1.80%
Maximum | Default Rate [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Constant Default Rate	8.75%
Standard & Poor's, B Rating [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Basis points	2.50%
Standard & Poor's, CCC Rating [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Basis points	2.50%

Goodwill and Other Intangible70

Goodwill and Other Intangible Assets (Details) ₪ in Millions, $ in Millions	2 Months Ended		6 Months Ended
	Feb. 23, 2015ILS (₪)	Feb. 23, 2015USD ($)	Jun. 30, 2015USD ($)	Jun. 30, 2014USD ($)	Mar. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Mar. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Acquired Finite-Lived Intangible Assets [Line Items]
Payments to Acquire Businesses and Interest in Affiliates			$ 2.4	$ 32.2
Assets			10,429.3			$ 10,455.7
Goodwill			169.2	23.8	$ 169.2	168.9	$ 23.8	$ 0
Intangible Assets, Net (Excluding Goodwill)			186.4	$ 59.8	$ 193.4	197.5	$ 62.4	$ 20.7
Liabilities			$ 5,970.8			$ 5,917.3
Cashboard [Member] | Common Stock
Acquired Finite-Lived Intangible Assets [Line Items]
Ownership interest (as a percent)		56.20%	68.30%
Common Stock | Israel, New Shekels | Cashboard [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪	₪ 9.5
Common Stock | United States of America, Dollars | Cashboard [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Payments to Acquire Businesses and Interest in Affiliates		$ 2.4
Cashboard [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Assets		5.5
Goodwill		0.3
Intangible Assets, Net (Excluding Goodwill)		2.8
Liabilities		$ 1.2

Goodwill and Other Intangible71

Goodwill and Other Intangible Assets - Goodwill Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 169.2	$ 23.8	$ 169.2	$ 23.8	$ 169.2	$ 168.9	$ 23.8	$ 0
Intangible Assets, Net (Excluding Goodwill)	186.4	59.8	186.4	59.8	$ 193.4	$ 197.5	$ 62.4	$ 20.7
Goodwill, Acquired During Period	0	0	0.3	23.8
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill	0	0	2.8	42.4
Disposition of Goodwill through disposition of business	0	0	0	(0.4)
Amortization of Intangible Assets	$ (7)	$ (2.6)	$ (13.9)	$ (2.9)

Debt (Details)

Debt (Details)	3 Months Ended	6 Months Ended
	Jun. 30, 2015USD ($)	Jun. 30, 2015USD ($)	Jun. 30, 2014USD ($)	Dec. 31, 2014USD ($)	Nov. 25, 2014USD ($)	Oct. 10, 2014USD ($)	Aug. 14, 2013USD ($)
Debt Instrument
Total debt	$ 748,500,000	$ 748,500,000		$ 746,600,000
Repayments of Debt		17,600,000	$ 200,000
Draw down of debt and revolving line of credit		19,500,000	$ 0
WTM Bank Facility
Debt Instrument
Total commitment under revolving credit facility							$ 425,000,000
Proceeds from Issuance of Debt		$ 15,000,000
Debt, Weighted Average Interest Rate	3.65%	3.65%
Repayments of Debt		$ 15,000,000
Other Debt [Member]
Debt Instrument
Total debt	$ 4,800,000	4,800,000		4,800,000
OneBeacon | 2012 OBH Senior Notes
Debt Instrument
Debt instrument at face value	275,000,000	275,000,000		275,000,000
Unamortized original issue discount	(300,000)	(300,000)		(300,000)
Total debt	274,700,000	274,700,000		274,700,000
Sirius Group | SIG Senior Notes
Debt Instrument
Debt instrument at face value	400,000,000	400,000,000		400,000,000
Unamortized original issue discount	(300,000)	(300,000)		(300,000)
Total debt	399,700,000	399,700,000		399,700,000
White Mountains | WTM Bank Facility
Debt Instrument
Total debt	0	0		0
Tranzact [Member] | Tranzact Bank Facility [Member]
Debt Instrument
Long-term Line of Credit	70,600,000	70,600,000		68,700,000
Unamortized Debt Issuance Expense	(1,300,000)	(1,300,000)		(1,300,000)
Line of Credit Facility, Fair Value of Amount Outstanding	69,300,000	69,300,000		$ 67,400,000
Total commitment under revolving credit facility						$ 82,000,000
Notional amount	66,100,000	66,100,000				70,000,000
Line of Credit Facility, Remaining Borrowing Capacity						$ 12,000,000
Proceeds from Long-term Lines of Credit		4,500,000
Repayments of Debt	$ 1,300,000	$ 2,600,000
Sirius International [Member] | Letter of Credit [Member]
Debt Instrument
number of Credit Lines					2
Line of Credit Facility, Fair Value of Amount Outstanding					$ 200,000,000
Nordea Bank Finland
Debt Instrument
Total commitment under revolving credit facility					125,000,000
Lloyds Bank plc [Member]
Debt Instrument
Total commitment under revolving credit facility					75,000,000
Unsecured Debt [Member] | Nordea Bank Finland
Debt Instrument
Total commitment under revolving credit facility					100,000,000
Unsecured Debt [Member] | Lloyds Bank plc [Member]
Debt Instrument
Total commitment under revolving credit facility					$ 25,000,000

Income Taxes (Details)

Income Taxes (Details) - Entity [Domain] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jul. 17, 2013
Income Tax Contingency [Line Items]
Effective tax rates (as a percent)	46.60%	22.40%	27.50%	25.00%
U.S. statutory rate (as a percent)			35.00%	35.00%
Income Tax Expense (Benefit)	$ (2.7)	$ 24.5	$ 22.3	$ 55.4
Tax Assessment Year 2007 to 2009 | Internal Revenue Service (IRS)
Income Tax Contingency [Line Items]
Total tax assessment					$ 74.8
Tax assessments related to disallowed deductions					$ 60.2

Derivatives (Details)

Derivatives (Details) ¥ in Billions, $ in Billions	Jun. 30, 2015USD ($)	Jun. 30, 2015JPY (¥)	Dec. 31, 2014USD ($)	Dec. 31, 2014JPY (¥)
Collective account values of underlying variable annuities as a percentage of the guarantee value			113.00%	113.00%
White Mountains Life Re
Notional amount	$ 0.8	¥ 100.7	$ 1.1	¥ 134.2
Collective account values of underlying variable annuities as a percentage of the guarantee value	113.00%	113.00%

Derivatives Pre-Tax Operating R

Derivatives Pre-Tax Operating Results of WM Life Re (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Variable Annuity Reinsurance [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 6.8	$ 12.5	$ 14.1	$ 26.3
Pre-tax operating results of WM Life Re
Total revenues	587.5	653.3	1,244.3	1,232
General and administrative expenses	(108.1)	(70.3)	(224.5)	(120.6)
Pre-tax income from continuing operations	(5.8)	109.5	81	222
White Mountains Life Re
Pre-tax operating results of WM Life Re
Fees, included in other revenue	2.6	5.1	5.6	10.5
Change in fair value of variable annuity liability, included in other revenue	0.7	22	1.1	20.7
Realized and unrealized gains (losses)	(2.3)	(26.2)	(7.8)	(32.7)
Foreign exchange, included in other revenue	0	0.2	(1.2)	0.5
Other investment income and (losses) gains	(0.1)	0.2	(0.2)	0.5
Total revenues	0.9	1.3	(2.5)	(0.5)
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	0	0.3	0	0.3
Death benefit claims paid, included in general and administrative expenses	(0.1)	(0.1)	0	(0.1)
General and administrative expenses	(0.9)	(1)	(2.3)	(2.4)
Pre-tax income from continuing operations	$ (0.1)	$ 0.5	$ (4.8)	$ (2.7)

Derivatives Realized and Unreal

Derivatives Realized and Unrealized Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Variable Annuity Reinsurance [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 6.8	$ 12.5	$ 14.1	$ 26.3
White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(2.3)	(26.2)	(7.8)	(32.7)
Derivative, fair value, net	35.7	74.8	35.7	74.8	$ 47	$ 56.4	$ 89.6	$ 69.2
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0		0
Derivative Transfers In (Out), Net	0	0	0	0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	9	(11.4)	12.9	(38.3)
Fixed income/interest rate | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(5.9)	(5.6)	3.4	(12)
Derivative, fair value, net	(0.1)		(0.1)			(1.7)
Foreign exchange | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	9.2	(6.4)	(2.9)	(12.9)
Derivative, fair value, net	29		29			44.1
Equity | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(5.6)	(14.2)	(8.3)	(7.8)
Derivative, fair value, net	6.8		6.8			14
Level 2 Inputs | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	0.6	(15.5)	(5.9)	(22.5)
Derivative, fair value, net	32.5	22.3	32.5	22.3	31.2	33.8	25.1	4.7
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0		0
Derivative Transfers In (Out), Net	0	0	0	0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	(0.7)	(12.7)	(4.6)	(40.1)
Level 1 Inputs | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	5.4	(0.1)	7.1	(0.3)
Derivative, fair value, net	(5.1)	(0.8)	(5.1)	(0.8)	(1.2)	3.7	0.4	1.1
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0		0
Derivative Transfers In (Out), Net	0	0	0	0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	9.3	1.1	15.9	1.6
Fair Value, Inputs, Level 3 [Member] | Variable Annuity | White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	1.8	(31.8)	1.8	(31.8)	1.1	0.7	(54.1)	(52.8)
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	0.7	22.3	1.1	21
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers	0	0	0	0
Fair Value Measurement with Unobservable Inputs, Reconciliation, Recurring Basis, Liability Sales or Settlements	0	0	0	0
Fair Value, Inputs, Level 3 [Member] | Derivative [Member] | White Mountains Life Re
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Realized and unrealized gains (losses)	(8.3)	(10.6)	(9)	(9.9)
Derivative, fair value, net	8.3	53.3	8.3	53.3	$ 17	$ 18.9	$ 64.1	$ 63.4
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0		0
Derivative Transfers In (Out), Net	0	0	0	0
Fair Value Derivatives Measured on Recurring Basis, Asset Sales and Settlements	$ 0.4	$ 0.2	$ 1.6	$ 0.2

Derivatives Variable Annuity Re

Derivatives Variable Annuity Reinsurance Liabilities and Derivative Instruments (Details) $ in Millions, ¥ in Billions	3 Months Ended		6 Months Ended
	Jun. 30, 2015USD ($)	Jun. 30, 2014USD ($)	Jun. 30, 2015USD ($)	Jun. 30, 2014USD ($)	Jun. 30, 2015JPY (¥)	Dec. 31, 2014USD ($)	Dec. 31, 2014JPY (¥)
Changes in derivative instruments
Collective account values of underlying variable annuities as a percentage of the guarantee value						113.00%	113.00%
White Mountains Life Re
Variable Annuity Reinsurance [Line Items]
Notional amount	$ 800		$ 800		¥ 100.7	$ 1,100	¥ 134.2
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0		0
Changes in derivative instruments
Beginning balance	47	$ 89.6	56.4	$ 69.2
Purchases		0		0
Realized and unrealized gains (losses)	(2.3)	(26.2)	(7.8)	(32.7)
Transfers in	0	0	0	0
Sales/settlements	(9)	11.4	(12.9)	38.3
Ending balance	35.7	74.8	35.7	74.8
Change in fair value of variable annuity death benefit liabilities, included in other general and administrative expenses	$ 0	0.3	$ 0	0.3
Collective account values of underlying variable annuities as a percentage of the guarantee value	113.00%		113.00%		113.00%
White Mountains Life Re | Level 2 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	$ 0		$ 0
Changes in derivative instruments
Beginning balance	31.2	25.1	33.8	4.7
Purchases		0		0
Realized and unrealized gains (losses)	0.6	(15.5)	(5.9)	(22.5)
Transfers in	0	0	0	0
Sales/settlements	0.7	12.7	4.6	40.1
Ending balance	32.5	22.3	32.5	22.3
White Mountains Life Re | Level 1 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0		0
Changes in derivative instruments
Beginning balance	(1.2)	0.4	3.7	1.1
Purchases		0		0
Realized and unrealized gains (losses)	5.4	(0.1)	7.1	(0.3)
Transfers in	0	0	0	0
Sales/settlements	(9.3)	(1.1)	(15.9)	(1.6)
Ending balance	(5.1)	(0.8)	(5.1)	(0.8)
Variable Annuity | White Mountains Life Re | Level 3 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0		0
Changes in derivative instruments
Beginning balance	1.1	(54.1)	0.7	(52.8)
Realized and unrealized gains (losses)	0.7	22.3	1.1	21
Transfers in	0	0	0	0
Sales/settlements	0	0	0	0
Ending balance	1.8	(31.8)	1.8	(31.8)
Purchases		0		0
Derivative [Member] | White Mountains Life Re | Level 3 Inputs
Variable Annuity Reinsurance [Line Items]
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases	0		0
Changes in derivative instruments
Beginning balance	17	64.1	18.9	63.4
Purchases		0		0
Realized and unrealized gains (losses)	(8.3)	(10.6)	(9)	(9.9)
Transfers in	0	0	0	0
Sales/settlements	(0.4)	(0.2)	(1.6)	(0.2)
Ending balance	$ 8.3	$ 53.3	$ 8.3	$ 53.3

Derivatives Additional Collater

Derivatives Additional Collateral (Details) - White Mountains Life Re - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Credit risk on derivative instruments
Collateral cash and investments	$ 25.3	$ 33.2	$ 42.3
Cash
Credit risk on derivative instruments
Derivative collateral right to reclaim	20.9	23.7	27.8
Fixed maturity investments.
Credit risk on derivative instruments
Derivative collateral right to reclaim	$ 4.4	$ 9.5	$ 14.5

Derivatives (Level 3 Derivative

Derivatives (Level 3 Derivative Instruments) (Details) - Jun. 30, 2015 - White Mountains Life Re - USD ($) $ in Millions	Total
Discounted cash flows
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	$ (1.8)
Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Mortality (as a percent)	0.00%
Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Mortality (as a percent)	6.40%
Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Mortality (as a percent)	1.10%
Foreign Exchange Option [Member] | Counterparty valuations
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	$ 2.7
Foreign Exchange Option [Member] | Counterparty valuations | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	0.50%
Foreign Exchange Option [Member] | Counterparty valuations | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	11.60%
Foreign Exchange Option [Member] | Counterparty valuations | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Foreign exchange volatilities (as a percent)	5.30%
Equity Index Options | Counterparty valuations
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	$ 5.6
Equity Index Options | Counterparty valuations | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	(0.70%)
Equity Index Options | Counterparty valuations | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	6.50%
Equity Index Options | Counterparty valuations | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Expected equity dividends (as a percent)	1.50%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	0.10%
Foreign exchange volatilities (as a percent)	10.60%
Index volatilities (as a percent)	24.50%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	40.00%
Foreign exchange volatilities (as a percent)	16.10%
Index volatilities (as a percent)	27.90%
Year One | Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average
Quantitative information for Level 3 Fair Value Measurements Liabilities
Surrenders (as a percent)	40.00%
Foreign exchange volatilities (as a percent)	12.80%
Index volatilities (as a percent)	26.30%

Derivatives (Value, Collateral

Derivatives (Value, Collateral (held) provided by WM Life Re and Net Exposure to Credit Losses on OTC Derivative Instruments) (Details) - White Mountains Life Re - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Offsetting Liabilities [Line Items]
Derivative Assets	$ 44.2	$ 64.4
Derivative Liabilities	(8.4)	(8)
Net amounts recognized in Other assets	35.8	56.4
Over The Counter | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	2.2	1
Derivative Liabilities	(2.1)	(5.4)
Net amounts recognized in Other assets	0.1	(4.4)
Over The Counter | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	31.6	45.5
Derivative Liabilities	0	0
Net amounts recognized in Other assets	31.6	45.5
Over The Counter | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	10.2	11.7
Derivative Liabilities	(1.2)	(0.2)
Net amounts recognized in Other assets	9	11.5
Exchange Traded | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	0.2	2.8
Derivative Liabilities	(0.6)	(0.1)
Net amounts recognized in Other assets	(0.4)	2.7
Exchange Traded | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	0	0
Derivative Liabilities	(2.3)	(1.4)
Net amounts recognized in Other assets	(2.3)	(1.4)
Exchange Traded | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	0	3.4
Derivative Liabilities	(2.2)	(0.9)
Net amounts recognized in Other assets	$ (2.2)	$ 2.5

Derivatives (Uncollateralized A

Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) - White Mountains Life Re $ in Thousands	Jun. 30, 2015USD ($)Rating	Dec. 31, 2014USD ($)
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 35,800	$ 56,400
Collertal provided to counterparty in cash	5,300	3,500
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	41,100	59,900
Excess collateral provided to counterparty in cash	15,600	17,700
Derivative, Collateral, Right to Reclaim Securities	4,400	9,500
Counter-party collateral held by WMLife Re - Cash	11,400	9,900
Net amount of exposure to counter-party	$ 49,700	77,200
S and P Credit Ratings | Rating	23
Standard & Poor's, A Rating | Bank of America [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	$ 2,000	5,600
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	2,000	5,600
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	2,000	5,600
Standard & Poor's, A Rating | Barclays
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	0	100
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	0	100
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	0	100
Standard & Poor's, A Rating | Royal Bank of Scotland
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments		4,000
Collertal provided to counterparty in cash		0
Collateral provided to counterparty in financial instruments		0
Net amount of exposure after effect of collateral		4,000
Excess collateral provided to counterparty in cash		0
Derivative, Collateral, Right to Reclaim Securities		0
Counter-party collateral held by WMLife Re - Cash		0
Net amount of exposure to counter-party		4,000
Standard & Poor's, A Rating | Citigroup - OTC
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	21,300	22,200
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	21,300	22,200
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	4,900	1,100
Net amount of exposure to counter-party	16,400	21,100
Standard & Poor's, A Rating | Citigroup Exchange Traded [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	(5,000)	3,700
Collertal provided to counterparty in cash	5,000	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	0	3,700
Excess collateral provided to counterparty in cash	10,700	16,000
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	10,700	19,700
Standard & Poor's, A plus | JP Morgan
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	16,800	24,300
Collertal provided to counterparty in cash	0	0
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	16,800	24,300
Excess collateral provided to counterparty in cash	0	0
Derivative, Collateral, Right to Reclaim Securities	0	0
Counter-party collateral held by WMLife Re - Cash	6,500	8,800
Net amount of exposure to counter-party	10,300	15,500
Standard & Poor's, BBB plus Rating | Nomura [Member]
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	(300)	(3,500)
Collertal provided to counterparty in cash	300	3,500
Collateral provided to counterparty in financial instruments	0	0
Net amount of exposure after effect of collateral	0	0
Excess collateral provided to counterparty in cash	4,900	1,700
Derivative, Collateral, Right to Reclaim Securities	4,400	9,500
Counter-party collateral held by WMLife Re - Cash	0	0
Net amount of exposure to counter-party	9,300	$ 11,200
Standard & Poor's, A- Rating | Royal Bank of Scotland
Credit risk on derivative instruments
Excess collateral provided - Financial Instruments	1,000
Collertal provided to counterparty in cash	0
Collateral provided to counterparty in financial instruments	0
Net amount of exposure after effect of collateral	1,000
Excess collateral provided to counterparty in cash	0
Derivative, Collateral, Right to Reclaim Securities	0
Counter-party collateral held by WMLife Re - Cash	0
Net amount of exposure to counter-party	$ 1,000

Derivatives Forward Contracts (

Derivatives Forward Contracts (Details) - Forward Contracts SEK in Millions, $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2015USD ($)	Jun. 30, 2015USD ($)	Jun. 30, 2014USD ($)	Jun. 30, 2015SEK	Dec. 31, 2014USD ($)
Derivative Instruments, Gain (Loss) [Line Items]
Realized and unrealized gains (losses)	$ 0.4
Sirius Group
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	31.9	$ 31.9		SEK 262.7	$ 33.1
Realized and unrealized gains (losses)		0	$ (0.1)
Carrying Value	$ (0.2)	$ (0.2)			$ 0

Derivatives (Forward Contract C

Derivatives (Forward Contract Collateral) (Details) - Sirius Group SEK in Millions, $ in Millions	Jun. 30, 2015USD ($)Rating	Jun. 30, 2015SEKRating	Dec. 31, 2014USD ($)
Forward Contracts
Derivative [Line Items]
Notional amount	$ 31.9	SEK 262.7	$ 33.1
Carrying Value	$ (0.2)		0
S and P Credit Ratings | Rating	23	23
Interest Rate Cap
Derivative [Line Items]
S and P Credit Ratings | Rating	23	23
Standard & Poor's, A- Rating | Forward Contracts | Barclays
Derivative [Line Items]
Notional amount	$ 0.8		2.1
Carrying Value	0		0
Standard & Poor's, BBB plus Rating | Forward Contracts | Deutsche Bank
Derivative [Line Items]
Notional amount	8.5		0
Carrying Value	0		0
A | Forward Contracts | JP Morgan
Derivative [Line Items]
Notional amount	1.2		5.7
Carrying Value	0		0
Standard & Poor's, AA- Rating | Forward Contracts | Goldman Sachs
Derivative [Line Items]
Notional amount	5		8.7
Carrying Value	(0.1)		0
Standard & Poor's, AA- Rating | Forward Contracts | HSBC
Derivative [Line Items]
Notional amount	12		11.2
Carrying Value	(0.1)		0
Standard & Poor's, AA- Rating | Forward Contracts | Royal Bank of Canada
Derivative [Line Items]
Notional amount	4.4		5.4
Carrying Value	$ 0		$ 0

Derivatives (Interest rate cap)

Derivatives (Interest rate cap) (Details) - Investments [Domain] $ in Millions

1 Months Ended

3 Months Ended

6 Months Ended

Jul. 31, 2013USD ($)

May. 31, 2007shares

Jun. 30, 2015USD ($)Rating

Jun. 30, 2014USD ($)

Sep. 30, 2013

Jun. 30, 2015USD ($)Rating

Jun. 30, 2014USD ($)

Mar. 31, 2015USD ($)

Dec. 31, 2014USD ($)

Mar. 31, 2014USD ($)

Dec. 31, 2013USD ($)

Jul. 30, 2013

Interest Rate Cap

Derivative [Line Items]

Derivative collateral right to reclaim

$ 3.3

$ 3.3

Sirius Group Ltd [Member] | Preferred Stock

Derivative [Line Items]

Term of LIBOR loan

3 months

Basis points

3.20%

Sirius Group Ltd [Member] | Interest Rate Cap

Derivative [Line Items]

Basis points

3.95%

Sirius Group | Preferred Stock

Derivative [Line Items]

Preferred Stock, Dividend Rate, Percentage

7.506%

Sirius Group | Interest Rate Cap

Derivative [Line Items]

Derivative, Cap Interest Rate

8.30%

Derivative, Cost of Hedge

$ 9.9

years of interest rate cap

5 years

Derivative, fair value, net

3.3

$ 6.2

$ 3.3

$ 6.2

$ 3.1

$ 4.1

$ 8.2

$ 11.1

Realized and unrealized gains (losses)

$ 0.2

$ (2)

$ (0.8)

$ (4.9)

S and P Credit Ratings | Rating

23

23

Sirius Group | Interest Rate Cap | Preferred Stock

Derivative [Line Items]

Non-cumulative perpetual preference shares issued | shares

250,000,000

Standard & Poor's, A Rating | Barclays | Interest Rate Cap

Derivative [Line Items]

Derivative collateral right to reclaim

$ 2.3

$ 2.3

Standard & Poor's, AA- Rating | Nordea Bank Finland | Interest Rate Cap

Derivative [Line Items]

Derivative collateral right to reclaim

$ 1

$ 1

Derivatives (Weather Derivative

Derivatives (Weather Derivatives) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Oct. 10, 2014
Sirius Group | Weather derivatives
Derivative [Line Items]
Realized and unrealized gains (losses)	$ 2.3	$ 1	$ 3.1	$ 0.6
Tranzact [Member] | Tranzact Bank Facility [Member]
Derivative [Line Items]
Line of Credit Facility, Fair Value of Amount Outstanding	$ 66.1		$ 66.1		$ 70

Derivatives (Tranzact Interest

Derivatives (Tranzact Interest Rate Swap) (Details) - Tranzact [Member] - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Oct. 10, 2014
Tranzact Bank Facility [Member]
Derivative [Line Items]
Line of Credit Facility, Fair Value of Amount Outstanding	$ 69.3	$ 67.4
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	4.185%
Notional amount	$ 66.1		$ 70
Minimum | Tranzact Bank Facility [Member]
Derivative [Line Items]
LIBOR rate	4.00%
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative, Fixed Interest Rate	1.34%
Derivative, Variable Interest Rate	0.1866%		0.1518%
Debt Instrument, Interest Rate, Effective Percentage	5.50%
Derivative, fair value, net	$ (0.2)

Derivatives Foreign Currency Sw

Derivatives Foreign Currency Swap (Details) $ in Millions, ¥ in Billions	3 Months Ended		6 Months Ended
	Jun. 30, 2015USD ($)	Jun. 30, 2014USD ($)	Jun. 30, 2015USD ($)	Jun. 30, 2014USD ($)	Apr. 28, 2015USD ($)	Apr. 28, 2015JPY (¥)
HG Global
Derivative [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Domestic	$ 3.1	$ 5	$ 8.6	$ 9.6
Currency Swap [Member] | Sirius Group
Derivative [Line Items]
Derivative, fair value, net	(2.7)		(2.7)
Currency Swap [Member] | Sweden, Kronor | Sirius Group
Derivative [Line Items]
Notional amount					$ 50
Currency Swap [Member] | Sweden, Kronor | Sirius Group
Derivative [Line Items]
Notional amount | ¥						¥ 0.1
Surplus Note | BAM
Derivative [Line Items]
Interest Income, Other	$ 3.9	$ 4	$ 7.9	$ 7.9

Municipal Bond Guarantee (Detai

Municipal Bond Guarantee (Details) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 31, 2012USD ($)	Jun. 30, 2015USD ($)Contract	Jun. 30, 2014USD ($)	Jun. 30, 2015USD ($)yrContract	Jun. 30, 2014USD ($)	Dec. 31, 2014USD ($)Contract
BAM
Guarantor Obligations [Line Items]
Municipal bond contract | Contract		2,436		2,436		1,750
Insured Financial Obligations, Remaining Weighted Average Contract Period				12 years 8 months		12 years 10 months
Municipal bond outstanding principal		$ 17,766.6		$ 17,766.6		$ 12,362.5
Municipal bond outstanding interest		9,588.1		9,588.1		7,086.9
Municipal bond debt service outstanding		27,354.7		27,354.7		19,449.4
Financial Guarantee Insurance Contracts, Unearned Premium Revenue		37.1		$ 37.1		$ 27.6
Year of U.S. Treasury Rate | yr				1
HG Global
Guarantor Obligations [Line Items]
Partners Capital Contribution	$ 594.5
Surplus Notes	503
Income (Loss) from Continuing Operations before Income Taxes, Domestic		$ 3.1	$ 5	$ 8.6	$ 9.6
HG Global | Preferred Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)		96.90%		96.90%
HG Global | Common Stock
Guarantor Obligations [Line Items]
Ownership interest (as a percent)		88.40%		88.40%
Build America Mutual
Guarantor Obligations [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Domestic		$ (14.1)	(8)	$ (22.9)	(16.6)
HG Re
Guarantor Obligations [Line Items]
Statutory Accounting Practices, Statutory Capital and Surplus, Balance		$ 454.6		$ 454.6
BAM Management
Guarantor Obligations [Line Items]
Partners Capital Contribution	$ 14.5
Surplus Note | BAM
Guarantor Obligations [Line Items]
Basis points				3.00%
Derivative, Fixed Interest Rate		3.15%		3.15%		3.13%
Surplus Note | Build America Mutual
Guarantor Obligations [Line Items]
Interest income (expense), surplus notes		$ 3.9	$ 4	$ 7.9	$ 7.9
Combination One | Surplus Note | BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate | yr				5
Combination Two | Surplus Note | BAM
Guarantor Obligations [Line Items]
Number of years of change in interest rate | yr				3
Financial Guarantee [Member]
Guarantor Obligations [Line Items]
Percentage of par value of policy reinsured				15.00%
Fixed interest rate [Member] | Surplus Note | BAM
Guarantor Obligations [Line Items]
Derivative, Fixed Interest Rate		8.00%		8.00%

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’ common shareholders	$ 4.6	$ 92.9	$ 81	$ 188.9
Allocation of income for unvested restricted common shares	(0.1)	(1.2)	(0.9)	(2.3)
Dividends declared on participating restricted common shares	0	0	(0.1)	(0.1)
Total allocation to restricted common shares	(0.1)	(1.2)	(1)	(2.4)
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts	4.5	91.7	80	186.5
Undistributed net earnings (in millions):
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts	4.5	91.7	80	186.5
Dividends declared net of restricted common share amounts	0	0	(5.9)	(6.1)
Total undistributed net earnings, net of restricted common share amounts	$ 4.5	$ 91.7	$ 74.1	$ 180.4
Basic earnings per share denominators
Total average common shares outstanding during the period	5,985,700	6,162,200	5,982,100	6,166,000
Average unvested restricted common shares	(71,400)	(81,300)	(65,200)	(75,900)
Basic earnings per share denominator	5,914,300	6,080,900	5,916,900	6,090,100
Diluted earnings per share denominator
Total average common shares outstanding during the period	5,985,700	6,162,200	5,982,100	6,166,000
Average unvested restricted common shares	(71,400)	(81,300)	(65,200)	(75,900)
Average outstanding dilutive options to acquire common shares	0	0	0	0
Diluted earnings (loss) per share denominator	5,914,300	6,080,900	5,916,900	6,090,100
Basic earnings per share (in dollars):
Net income (loss) attributable to White Mountains' common shareholders	$ 0.76	$ 15.08	$ 13.53	$ 30.61
Dividends declared	(1)	(1)	(1)	(1)
Undistributed earnings (loss) (in dollars per share)	0.76	15.08	12.53	29.61
Earnings Per Share, Diluted [Abstract]
Net income (loss) attributable to White Mountains' common shareholders (in dollars per share)	0.76	15.08	13.53	30.61
Dividends declared	(1)	(1)	(1)	(1)
Undistributed earnings (loss) (in dollars per share)	$ 0.76	$ 15.08	$ 12.53	$ 29.61
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	125,000			125,000
Segment, Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Allocation of income for unvested restricted common shares	$ 0	$ 0	$ (0.1)	$ 0
Basic earnings per share denominators
Total average common shares outstanding during the period	5,985,700	6,162,200	5,982,100	6,166,000
Average unvested restricted common shares	(71,400)	(81,300)	(65,200)	(75,900)
Basic earnings per share denominator	5,914,300	6,080,900	5,916,900	6,090,100
Diluted earnings per share denominator
Total average common shares outstanding during the period	5,985,700	6,162,200	5,982,100	6,166,000
Average unvested restricted common shares	(71,400)	(81,300)	(65,200)	(75,900)
Average outstanding dilutive options to acquire common shares	0	0	0	0
Diluted earnings (loss) per share denominator	5,914,300	6,080,900	5,916,900	6,090,100
Earnings Per Share, Diluted [Abstract]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements				125,000

Non-controlling Interest (Detai

Non-controlling Interest (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 483.9	$ 542.7
Total Non-Controlling Excluding Reciprocals [Member]
Noncontrolling Interest [Line Items]
Total non-controlling interests	$ 125.1	$ 168.6
OneBeacon
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	24.70%	24.70%
Total non-controlling interests	$ 255	$ 258.4
SIG Preference Shares
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ 250	$ 250
HG Global
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	3.10%	3.10%
Total non-controlling interests	$ 17.5	$ 17.9
QuoteLab
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	40.00%	40.00%
Total non-controlling interests	$ 21.5	$ 22.6
Tranzact [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	36.80%	36.80%
Total non-controlling interests	$ 80.3	$ 88.2
WOBI
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	4.70%	36.70%
Total non-controlling interests	$ 2.3	$ 5.4
Prospector Offshore Fund Ltd [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	0.00%	23.40%
Total non-controlling interests	$ 0	$ 31.1
A W G Dewar Inc [Member]
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	31.90%	18.00%
Total non-controlling interests	$ 3.5	$ 3.4
Build America Mutual
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (131.8)	$ (121.9)
Star & Shield Insurance Exchange
Noncontrolling Interest [Line Items]
Noncontrolling equity percentage	100.00%	100.00%
Total non-controlling interests	$ (14.4)	$ (12.4)

Segment Information (Details)

Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment reporting information
Earned insurance and reinsurance premiums	$ 537.3	$ 503.4	$ 1,031.6	$ 997
Net investment income	23.8	29.6	47.1	54.3
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	(61.1)	113.9	42.3	177.7
Other revenue	87.5	6.4	123.3	3
Total revenues	587.5	653.3	1,244.3	1,232
Losses and LAE	288.3	282.7	553.7	512
Insurance and Reinsurance Acquisition Expenses	105.3	98.9	200.9	194
Other Underwriting Expense	80.7	81.9	162.2	163.3
General and administrative expenses	108.1	70.3	224.5	120.6
Interest expense	10.9	10	22	20.1
Total expenses	593.3	543.8	1,163.3	1,010
Pre-tax income (loss)	(5.8)	109.5	81	222
OneBeacon
Segment reporting information
Earned insurance and reinsurance premiums	319.3	290.9	605.9	567.4
Net investment income	10.1	12.2	20.6	22.6
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	(14)	21.9	0.2	40.8
Other revenue	(1.2)	1	(4.3)	2
Total revenues	314.2	326	622.4	632.8
Losses and LAE	194.5	186.1	360.4	335.5
Insurance and Reinsurance Acquisition Expenses	56.4	49.6	107.4	96.3
Other Underwriting Expense	52.9	51.7	108.8	101.1
General and administrative expenses	4.2	3.6	8.3	6.9
Interest expense	3.3	3.3	6.5	6.5
Total expenses	311.3	294.3	591.4	546.3
Pre-tax income (loss)	2.9	31.7	31	86.5
Sirius Group
Segment reporting information
Earned insurance and reinsurance premiums	215.5	209.6	420.5	425.1
Net investment income	9.3	11.8	17.3	20.5
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	(42.7)	58.4	37.2	86.5
Other revenue	5.4	(19.2)	(22)	(24.5)
Total revenues	187.5	260.6	453	507.6
Losses and LAE	92.1	91.4	189.6	170.2
Insurance and Reinsurance Acquisition Expenses	47.5	48.8	90.1	96.2
Other Underwriting Expense	27.7	30.2	53.2	62
General and administrative expenses	6.8	6.9	13	15.3
Interest expense	6.6	6.6	13.4	13.2
Total expenses	180.7	183.9	359.3	356.9
Pre-tax income (loss)	6.8	76.7	93.7	150.7
HG Global
Segment reporting information
Earned insurance and reinsurance premiums	0.6	0.3	1.1	0.5
Net investment income	0.5	0.3	0.9	0.6
Net investment income (loss) - surplus note interest	3.9	4	7.9	7.9
Net realized and unrealized investment gains (losses)	(1.4)	0.9	(0.3)	1.6
Other revenue	0	0	0	0
Total revenues	3.6	5.5	9.6	10.6
Losses and LAE	0	0	0	0
Insurance and Reinsurance Acquisition Expenses	0.1	0.1	0.2	0.2
Other Underwriting Expense	0	0	0	0
General and administrative expenses	0.4	0.4	0.8	0.8
Interest expense	0	0	0	0
Total expenses	0.5	0.5	1	1
Pre-tax income (loss)	3.1	5	8.6	9.6
BAM
Segment reporting information
Earned insurance and reinsurance premiums	0.1	0.1	0.3	0.2
Net investment income	0.8	1.4	1.8	2.8
Net investment income (loss) - surplus note interest	(3.9)	(4)	(7.9)	(7.9)
Net realized and unrealized investment gains (losses)	(1.9)	3.8	1.1	6.8
Other revenue	0.2	0.1	0.3	0.3
Total revenues	(4.7)	1.4	(4.4)	2.2
Losses and LAE	0	0	0	0
Insurance and Reinsurance Acquisition Expenses	0.6	0.5	1.4	0.9
Other Underwriting Expense	0.1	0.1	0.2	0.2
General and administrative expenses	8.7	8.8	16.9	17.7
Interest expense	0	0	0	0
Total expenses	9.4	9.4	18.5	18.8
Pre-tax income (loss)	(14.1)	(8)	(22.9)	(16.6)
Other Operations
Segment reporting information
Earned insurance and reinsurance premiums	1.8	2.5	3.8	3.8
Net investment income	3.1	3.9	6.5	7.8
Net investment income (loss) - surplus note interest	0	0	0	0
Net realized and unrealized investment gains (losses)	(1.1)	28.9	4.1	42
Other revenue	83.1	24.5	149.3	25.2
Total revenues	86.9	59.8	163.7	78.8
Losses and LAE	1.7	5.2	3.7	6.3
Insurance and Reinsurance Acquisition Expenses	0.7	(0.1)	1.8	0.4
Other Underwriting Expense	0	(0.1)	0	0
General and administrative expenses	88	50.6	185.5	79.9
Interest expense	1	0.1	2.1	0.4
Total expenses	91.4	55.7	193.1	87
Pre-tax income (loss)	$ (4.5)	$ 4.1	$ (29.4)	$ (8.2)

Investments in Unconsolidated92

Investments in Unconsolidated Affiliates (Investment in Unconsolidated Affiliates Table) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Investments in unconsolidated affiliates
Equity in earnings of unconsolidated affiliates, net of tax	$ 6.8	$ 12.5	$ 14.1	$ 26.3
Total investments in unconsolidated affiliates	397.2		397.2			$ 414.4
Symetra
Investments in unconsolidated affiliates
Equity in Unrealized Gains (Losses) from Investments in Unconsolidated Affiliates	2		2			37.6
Symetra | Common Stock
Investments in unconsolidated affiliates
Equity in earnings of unconsolidated affiliates, net of tax	6.1	13.1	13.3	27.5
Symetra Common Shares excluding unrealized gains (losses)	395.2		395.2			373.8
Total investments in unconsolidated affiliates	397.2	415.5	397.2	415.5	$ 447.3	411.4	$ 369	$ 317.3
Hamer, LLC [Member]
Investments in unconsolidated affiliates
Equity in earnings of unconsolidated affiliates, net of tax	1.1	$ 0.4	1.6	$ 0.8
Total investments in unconsolidated affiliates	$ 0		$ 0			$ 3

Investments in Unconsolidated93

Investments in Unconsolidated Affiliates (Symetra) (Details) - Entity [Domain] - Symetra - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2011
Investments in unconsolidated affiliates
Stock Repurchased and Retired During Period, Value	513,500
Shares of Symetra Financial Corporation owned (in shares)	20,562,379		20,562,379			20,048,879
Ownership interest (as a percent)	17.71%		17.71%			17.31%
Equity Method Investments Fair Value Per Share							$ 15
Term of amortization (in years)					30 years
Weight Average Years Remaining			28 years
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	$ 164.1		$ 164.1			$ 170.4
Common Stock
Investments in unconsolidated affiliates
Proceeds from Equity Method Investment, Dividends or Distributions	$ (2.2)	$ (2)	$ (4.4)	$ (4)
Fair value of investment							$ 261
Share Price	$ 24.17		$ 24.17
equity in earnings of unconsolidated affiliates
Investments in unconsolidated affiliates
After tax amortization through equity in earnings	$ 0.7	0.7	$ 1.4	1.4
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	33		33			36.4
Net unrealized gains (losses) of unconsolidated affiliates
Investments in unconsolidated affiliates
After tax amortization through equity in net unrealized losses	2.7	$ 2.9	5.5	$ 5.8
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	$ 131.1		$ 131.1			$ 134

Investments in Unconsolidated94

Investments in Unconsolidated Affiliates (Investments in Symetra Table Rollforward) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period			$ 414.4
Equity in earnings	$ 6.8	$ 12.5	14.1	$ 26.3
Carrying value at the end of the period	397.2		397.2
Income Tax Expense (Benefit)	$ (2.7)	24.5	$ 22.3	55.4
Symetra | Common Stock
Investments in unconsolidated affiliates
Share Price	$ 24.17		$ 24.17
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	$ 447.3	369	$ 411.4	317.3
Equity in earnings	6.1	13.1	13.3	27.5
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio	(66.4)	35.4	(35.5)	74.7
Dividends received	(2.2)	(2)	(4.4)	(4)
Carrying value at the end of the period	397.2	415.5	397.2	415.5
Aggregate value of White Mountains common shares of Symetra at quoted market price	497		497
Number of Shares Distributed	12.4	0	12.4	0
White Mountains | Common Stock
Changes in carrying value of investment in Symetra
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio			2	31.1
equity in earnings of unconsolidated affiliates | Symetra | Common Stock
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)	0.4	0.9	0.8	1.9
Net unrealized gains (losses) of unconsolidated affiliates | White Mountains | Common Stock
Changes in carrying value of investment in Symetra
Income Tax Expense (Benefit)			0	2.1
Amortization of common share basis difference | Symetra | Common Stock
Changes in carrying value of investment in Symetra
Equity in earnings	0.7	0.7	1.4	1.5
impairment of net unrealized gains (losses) | Net unrealized gains (losses) of unconsolidated affiliates | Symetra | Common Stock
Changes in carrying value of investment in Symetra
Equity in net unrealized gains (losses) from Symetra's fixed maturity portfolio	$ 2.8	$ 3.1	$ 5.7	$ 6.3

Investments in Unconsolidated95

Investments in Unconsolidated Affiliates Hamer (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Realized Gain (Loss) on Disposal	$ 20
Equity in earnings of unconsolidated affiliates, net of tax	6.8	$ 12.5	$ 14.1	$ 26.3
Hamer, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 1.1	$ 0.4	$ 1.6	$ 0.8

Employee Share-Based Incentiv96

Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Accrued Expense
Accrued expense at the beginning of period			$ 184.6
Accrued expense at the end of the period	$ 124.9		$ 124.9		$ 184.6
Performance Share | Range Of Performance Cycle From 2012 To 2014 [Member]
Accrued Expense
Vested (in shares)					3,570
Performance Share | W T M Incentive Plan | Range Of Performance Cycle From 2012 To 2014 [Member]
Accrued Expense
Percentage of target payout - minimum actual			91.00%
Percentage of target payout - maximum actual			145.50%
Performance Share | W T M Incentive Plan | Performance Cycle 2011 to 2013
Accrued Expense
Percentage of target payout - minimum actual					88.00%
Percentage of target payout - maximum actual					131.50%
White Mountains | Performance Share | W T M Incentive Plan
Target Performance Share Activity
Beginning of period (in shares)	116,396	127,537	123,549	119,220	119,220
Shares paid or expired (in shares)	0	0	(37,977)	(37,130)
New grants (in shares)	750	0	31,390	45,660
Assumed forfeitures and cancellations	(19)	0	165	(213)
Ending of period (in shares)	117,127	127,537	117,127	127,537	123,549
Accrued Expense
Accrued expense at the beginning of period	$ 27.6	$ 42.3	$ 48.1	$ 60.2	$ 60.2
Payments and deferrals	0	0	(27.3)	(26.7)
Assumed forfeitures and cancellations	(0.1)	(0.2)	0.5	0.2
Expense recognized	3.6	8.4	9.8	16.8
Accrued expense at the end of the period	31.1	$ 50.5	$ 31.1	$ 50.5	$ 48.1
Vesting Period			3 years
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 25.9		$ 25.9

Employee Share-Based Incentiv97

Employee Share-Based Incentive Compensation Plans (WTM - Performance Shares Granted Under the WTM Incentive Plan) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Share-based compensation arrangement by share-based payment award
Total Accrued Expense	$ 124.9		$ 184.6
White Mountains | Performance Share | W T M Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	120,130
Accrued Expense	$ 31.9
Target Performance Shares Outstanding, Assumed forfeitures	(3,003)
Accrued Expense, Assumed forfeitures	(0.8)
Total Target Performance Shares Outstanding	117,127	116,396	123,549	127,537	127,537	119,220
Total Accrued Expense	$ 31.1	$ 27.6	$ 48.1	$ 50.5	$ 42.3	$ 60.2
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2015 to 2017
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	31,390
Accrued Expense	$ 3.4
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2014 to 2016
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	41,937
Accrued Expense	$ 5.1
White Mountains | Performance Share | W T M Incentive Plan | Performance Cycle 2013 to 2015
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	46,803
Accrued Expense	$ 23.4

Employee Share-Based Incentiv98

Employee Share-Based Incentive Compensation Plans (WTM - Restricted Shares) (Details) - White Mountains - W T M Incentive Plan - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Restricted Share Cliff Vesting in January 2018
Target Restricted Share Activity
Issued (in shares)			23,640
Restricted Share Cliff Vesting in January 2017
Target Restricted Share Activity
Issued (in shares)			20,400
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)	71,125	81,325	83,314	94,130	94,130
Issued (in shares)	750	0	23,640	20,400
Vested (in shares)	0	0	(35,079)	(33,205)
Assumed forfeitures and cancellations	0	0	0	0
Ending of period (in shares)	71,875	81,325	71,875	81,325	83,314
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance	$ 26.5	$ 24.8	$ 14.3	$ 17	$ 17
Issued	0.5	0	15.7	11.8
Assumed forfeitures and cancellations	0	0	0	0
Expense recognized	(3.8)	(3.9)	(6.8)	(7.9)
Unamortized grant date fair value, ending balance	$ 23.2	$ 20.9	$ 23.2	$ 20.9	$ 14.3

Employee Share-Based Incentiv99

Employee Share-Based Incentive Compensation Plans (OneBeason - Peformance Shares) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Expense Amortized
Accrued expense at the beginning of period			$ 184.6
Accrued expense at the end of the period	$ 124.9		$ 124.9		$ 184.6
OneBeacon | Performance Share | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Vesting Period			3 years
OneBeacon | One Beacon Long Term Incentive Plan | Range Of Performance Cycle From 2012 To 2014 [Member]
Expense Amortized
Percentage of target payout - minimum actual			45.70%
OneBeacon | One Beacon Long Term Incentive Plan | Performance Cycle 2011 to 2013
Expense Amortized
Percentage of target payout - minimum actual			37.10%
OneBeacon | One Beacon Long Term Incentive Plan | Performance Share
Target Performance Shares Outstanding
Beginning of period (in shares)	462,147	512,938	517,470	493,421	493,421
Shares paid or expired (in shares)	0	0	(181,290)	(142,138)
New grants (in shares)	0	0	154,887	165,800
Assumed forfeitures and cancellations	0	0	(28,920)	(4,145)
Ending of period (in shares)	462,147	512,938	462,147	512,938	517,470
Expense Amortized
Accrued expense at the beginning of period	$ 2.1	$ 3.6	$ 3.4	$ 4	$ 4
Payments and deferrals	0	0	(1.5)	(1)
Assumed forfeitures and cancellations	0	0	0	0
Expense recognized	0.3	0.8	0.5	1.4
Accrued expense at the end of the period	2.4	$ 4.4	2.4	$ 4.4	$ 3.4
Additional compensation cost that would be recognized if all outstanding performance shares vested	$ 2.8		$ 2.8

Employee Share-Based Incenti100

Employee Share-Based Incentive Compensation Plans (OneBeacon - Performance Shares Outstanding) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Share-based compensation arrangement by share-based payment award
Accrued incentive compensation	$ 124.9		$ 184.6
OneBeacon | Performance Share | One Beacon Long Term Incentive Plan
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	473,997
Incentive Compensation Payable before Forfeitures	$ 2.4
Target Performance Shares Outstanding, Assumed forfeitures	(11,850)
Accrued Expense, Assumed forfeitures	0
Total Target Performance Shares Outstanding	462,147	462,147	517,470	512,938	512,938	493,421
Accrued incentive compensation	$ 2.4	$ 2.1	$ 3.4	$ 4.4	$ 3.6	$ 4
OneBeacon | Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2015 to 2017
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	154,887
Incentive Compensation Payable before Forfeitures	$ 0.3
OneBeacon | Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2014 to 2016
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	151,810
Incentive Compensation Payable before Forfeitures	$ 0.6
OneBeacon | Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2013 to 2015
Share-based compensation arrangement by share-based payment award
Target Performance Shares Outstanding	167,300
Incentive Compensation Payable before Forfeitures	$ 1.5

Employee Share-Based Incenti101

Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Shares) (Details) - OneBeacon - One Beacon Long Term Incentive Plan $ in Millions	Feb. 24, 2015shares	Mar. 01, 2012shares	May. 31, 2011shares	Jun. 30, 2015USD ($)shares	Jun. 30, 2014USD ($)shares	Jun. 30, 2015USD ($)shares	Jun. 30, 2014USD ($)shares	Dec. 31, 2014USD ($)shares	Feb. 28, 2012Installment	May. 25, 2011Installment
Restricted Stock
Target Restricted Share Activity
Beginning of period (in shares)				390,950	612,500	612,500	915,000	915,000
Issued (in shares)	75,950	300,000		0	0	75,950	0
Vested (in shares)				0	0	(296,000)	(300,000)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value | $						$ 0
Assumed forfeitures and cancellations				0	0	(1,500)	(2,500)
Ending of period (in shares)				390,950	612,500	390,950	612,500	612,500
Unamortized grant date fair value
Unamortized grant date fair value, beginning balance | $				$ 4	$ 5.7	$ 3.5	$ 6.5	$ 6.5
Issued | $				0	0	1.1	0
Assumed forfeitures and cancellations | $				0	0	0	0
Expense recognized | $				(0.5)	(0.8)	(1.1)	(1.6)
Unamortized grant date fair value, ending balance | $				$ 3.5	$ 4.9	$ 3.5	$ 4.9	$ 3.5
Number of Installments in which the Shares Vest | Installment									2
Restricted Stock | Chief Executive Officer
Target Restricted Share Activity
Issued (in shares)			630,000
Unamortized grant date fair value
Number of Installments in which the Shares Vest | Installment										4
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years			35,000
Performance Share
Target Restricted Share Activity
Beginning of period (in shares)				462,147	512,938	517,470	493,421	493,421
Assumed forfeitures and cancellations				0	0	(28,920)	(4,145)
Ending of period (in shares)				462,147	512,938	462,147	512,938	517,470
Unamortized grant date fair value
Assumed forfeitures and cancellations | $				$ 0	$ 0	$ 0	$ 0
Expense recognized | $				0.3	$ 0.8	0.5	$ 1.4
Unamortized grant date fair value, ending balance | $				$ 2.8		$ 2.8
Performance Share | Chief Executive Officer
Unamortized grant date fair value
Annual reduction in performance shares			35,000
Years Performance Shares will be Forfeited			5 years

Employee Share-Based Incenti102

Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Stock Units) (Details) - Jun. 30, 2015 - Restricted Stock Units (RSUs) [Member] - OneBeacon - One Beacon Long Term Incentive Plan - USD ($) $ in Millions	Total	Total
Share-based compensation arrangement by share-based payment award
Issued (in shares)		214,430
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	209,369	209,369
Expense recognized	$ 0.3	$ 0.4

Fair Value of Financial Inst103

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	$ 748.5	$ 746.6
OneBeacon | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	274.7	274.7
OneBeacon | Book Value | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	274.7	274.7
OneBeacon | Estimate of Fair Value Measurement [Member] | 2012 OBH Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	280.4	286
Sirius Group | Book Value | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	399.7	399.7
Sirius Group | Book Value | SIG Preference Shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	250	250
Sirius Group | Estimate of Fair Value Measurement [Member] | SIG Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	437.8	437.8
Sirius Group | Estimate of Fair Value Measurement [Member] | SIG Preference Shares
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	261.8	260
Tranzact [Member] | Tranzact Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Long-term Line of Credit	70.6	68.7
Tranzact [Member] | Estimate of Fair Value Measurement [Member] | Tranzact Bank Facility [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Long-term Line of Credit	$ 70.2	$ 68.7

Discontinued Operations (Detail

Discontinued Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Discontinued Operations [Line Items}
Net (loss) income from sale of discontinued operations	$ (0.1)	$ 2.7	$ 7.9	$ 2.7
Net loss from discontinued operations, net of tax	(0.2)	(0.1)	(0.3)	(0.6)
Other revenue	87.5	6.4	123.3	3
Segment, Discontinued Operations
Discontinued Operations [Line Items}
Net (loss) income from sale of discontinued operations	0.1	2.7	8	2.7
Esurance | Segment, Discontinued Operations
Discontinued Operations [Line Items}
Income Taxes Paid	0.3
OneBeacon Runoff [Member]
Discontinued Operations [Line Items}
Net (loss) income from sale of discontinued operations	5.3			5.3
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax	8.2			8.2
Reserve charge under the SPA, before tax		7.4	$ 7.4
Reserve Charge After-Tax		$ 4.8		$ 4.8
OneBeacon Runoff [Member] | Segment, Discontinued Operations
Discontinued Operations [Line Items}
Net (loss) income from sale of discontinued operations	$ 0.3

Discontinued Operations - Earni

Discontinued Operations - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	$ (0.1)	$ (1.2)	$ (0.9)	$ (2.3)
Net Income (Loss) Available to Common Stockholders, Basic	$ 4.3	$ 95.5	$ 88.6	$ 191
Total average common shares outstanding during the period	5,985,700	6,162,200	5,982,100	6,166,000
Weighted Average Number of Shares, Restricted Stock	(71,400)	(81,300)	(65,200)	(75,900)
Basic earnings per share denominator	5,914,300	6,080,900	5,916,900	6,090,100
Average outstanding dilutive options to acquire common shares	0	0	0	0
Diluted earnings (loss) per share denominator	5,914,300	6,080,900	5,916,900	6,090,100
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	125,000			125,000
Segment, Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net loss from discontinued operations, net of tax	$ (0.3)	$ 2.6	$ 7.6	$ 2.1
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	0	0	(0.1)	0
Net Income (Loss) Available to Common Stockholders, Basic	$ (0.3)	$ 2.6	$ 7.5	$ 2.1
Total average common shares outstanding during the period	5,985,700	6,162,200	5,982,100	6,166,000
Weighted Average Number of Shares, Restricted Stock	(71,400)	(81,300)	(65,200)	(75,900)
Basic earnings per share denominator	5,914,300	6,080,900	5,916,900	6,090,100
Average outstanding dilutive options to acquire common shares	0	0	0	0
Diluted earnings (loss) per share denominator	5,914,300	6,080,900	5,916,900	6,090,100
Earnings Per Share, Basic and Diluted	$ (0.04)	$ 0.43	$ 1.27	$ 0.34
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements				125,000

Contingencies (Details)

Contingencies (Details) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2007USD ($)	Jun. 30, 2015USD ($)	Jun. 30, 2014USD ($)	Jun. 30, 2015USD ($)	Jun. 30, 2014USD ($)	Jan. 22, 2015	Oct. 07, 2011
Loss Contingencies [Line Items]
Net (loss) income from sale of discontinued operations		$ (0.1)	$ 2.7	$ 7.9	$ 2.7
Esurance
Loss Contingencies [Line Items]
Percentage of fluctuation in loss reserves of transferred subsidiaries retained							90.00%
Tribune Company Litigation | Pending Litigation
Loss Contingencies [Line Items]
Proceeds from Sale, Maturity and Collection of Investments	$ 39
Segment, Discontinued Operations
Loss Contingencies [Line Items]
Net (loss) income from sale of discontinued operations		0.1	$ 2.7	8	2.7
Gain Contingency, Unrecorded Amount		18		$ 18
OneBeacon Runoff [Member]
Loss Contingencies [Line Items]
Net (loss) income from sale of discontinued operations		5.3			$ 5.3
Number of insurance holders						3
OneBeacon Runoff [Member] | Segment, Discontinued Operations
Loss Contingencies [Line Items]
Net (loss) income from sale of discontinued operations		$ 0.3

Subsequent Events (Details)

Subsequent Events (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Jul. 27, 2015
Subsequent Event [Line Items]
Other Underwriting Expense		$ 80.7	$ 81.9	$ 162.2	$ 163.3
UNITED STATES | Sale of Sirius [Member]
Subsequent Event [Line Items]
Stop Loss Limitation				45
Industry Loss Trigger on Industry Loss Warranty				15,000
UNITED STATES | Combination One | Sale of Sirius [Member]
Subsequent Event [Line Items]
Stop Loss Limitation				75
Industry Loss Trigger on Industry Loss Warranty				40,000
UNITED STATES | Combination Two | Sale of Sirius [Member]
Subsequent Event [Line Items]
Stop Loss Limitation				22.5
Industry Loss Trigger on Industry Loss Warranty				50,000
JAPAN | Sale of Sirius [Member]
Subsequent Event [Line Items]
Stop Loss Limitation				25
Industry Loss Trigger on Industry Loss Warranty				12,500
Sirius Group | Sale of Sirius [Member]
Subsequent Event [Line Items]
Percentage of common shareholder's equity							127.30%
Additional cash that will be paid as part of the purchase price							$ 10
Payments for (Proceeds from) Businesses and Interest in Affiliates				2,200
OneBeacon | Crop-hail insurance [Member]
Subsequent Event [Line Items]
Years of Agreement	5 years
Proceeds from Divestiture of Businesses and Interests in Affiliates	$ 3
Percentage of Quota Share Reinsurance Agreement	100.00%
Premiums Written, Net				26.6		$ 35.1
Underwriting Income (Loss)				(1.3)		(3.5)
Other Underwriting Expense				$ 1.5		$ 1

Uncategorized Items - wtm-20150

Label	Element	Value
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	us-gaap_ProfitLoss	$ 100.1
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	us-gaap_ProfitLoss	$ 3.4