Investments in Unconsolidated Affiliates (Details 3) (USD $) In Millions, except Per Share data	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period			$ 389.7
Equity in earnings of Symetra	(1.5)	(7.9)	(16.1)	(1.6)
Carrying value at the end of the period	458.6		458.6
Symetra
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	421.0	372.5	387.5	307.7
Equity in earnings of Symetra	1.6	8.3	17.5	2.1
Net unrealized gains from investee's fixed maturity portfolio	59.9	46.9	84.0	129.7
Dividends received	(1.0)	(0.9)	(2.9)	(1.8)
(Decrease) increase in value of warrants	(24.8)	(10.7)	(29.4)	(21.6)
Carrying value at the end of the period	456.7	416.1	456.7	416.1
Tax on net unrealized gains from investee's fixed maturity portfolio	4.8	3.8	6.8	3.8
Aggregate value of White Mountains common shares of Symetra at quoted market price	141.8		141.8
Quoted market price (in dollars per share)	$ 8.15		$ 8.15
Symetra \| Common shareholders' equity
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	388.5	344.9	350.4	269.2
Equity in earnings of Symetra	1.6	8.3	17.5	2.1
Net unrealized gains from investee's fixed maturity portfolio	59.9	46.9	84.0	129.7
Dividends received	(1.0)	(0.9)	(2.9)	(1.8)
Carrying value at the end of the period	449.0	399.2	449.0	399.2
Tax on equity in earnings	0.1	0.7	1.4	0.7
Tax on net unrealized gains from investee's fixed maturity portfolio	4.8	3.8	6.8	3.8
Equity in net unrealized gains	147.7	120.7	147.7	120.7
Tax on equity in net unrealized gains	12.3	9.8	24.2	9.8
Loss from dilutive effect resulting from public offering				17.9
Loss from dilutive effect resulting from dividend and issuance of restricted shares		0.1		0.7
Gain from dilutive effect resulting from public offering				1.9
Symetra \| Warrants
Changes in carrying value of investment in Symetra
Carrying value at the beginning of the period	32.5	27.6	37.1	38.5
(Decrease) increase in value of warrants	(24.8)	(10.7)	(29.4)	(21.6)
Carrying value at the end of the period	7.7	16.9	7.7	16.9
Cash dividends received on warrants	$ 0.6		$ 1.6

This item represents disclosure of the amount of dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporation; these investments are accounted for under the equity method of accounting. This element excludes distributions that constitute a return of investment, which are classified as investing activities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 28

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 18

-Paragraph 19

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 5

-Section 03

-Paragraph 13