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Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended | 3 Months Ended |  | 12 Months Ended |  | 3 Months Ended | 1 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Mar. 31, 2012 Restricted Share Cliff Vesting in January 2015 | Sep. 30, 2012 Restricted Share Cliff Vesting in January 2015 | Dec. 31, 2012 Restricted Share Cliff Vesting in July 2015 | Dec. 31, 2012 Restricted Stock | Dec. 31, 2011 Restricted Stock | Dec. 31, 2010 Restricted Stock | Mar. 31, 2007 Restricted Stock Chief Executive Officer | Dec. 31, 2010 Restricted Stock Chief Executive Officer Installment | Dec. 31, 2012 Restricted Share Vesting Beginning in February 2014 | Mar. 31, 2012 Restricted Share Vesting Beginning in February 2014 Installment | Jun. 30, 2011 Restricted Share Cliff Vesting in January 2014 | Mar. 31, 2011 Restricted Share Cliff Vesting in January 2014 | Mar. 31, 2010 Restricted Share Vested in December 2012 | May 31, 2010 Non-Qualified Options
Target Performance Share activity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning of period (in shares) |  |  |  |  |  |  | 72,000 |  | 46,250 |  | 91,900 |  |  |  |  |  |  |  |  |
Issued (in shares) | 32,160 | 27,250 | 19,750 | 25,460 | 2,500 | 1,200 | 32,160 |  | 27,250 |  | 19,750 |  | 15,000 |  | 3,000 |  | 250 | 27,000 | 19,750 |
Vested (in shares) |  |  |  |  |  |  | (32,945) |  | (1,500) |  | (65,150) |  |  |  |  |  |  |  |  |
Assumed forfeitures and cancellations (in shares) |  |  |  |  |  |  | (1,305) |  | 0 |  | (250) |  |  |  |  |  |  |  |  |
Ending of period (in shares) |  |  |  |  |  |  | 69,910 |  | 72,000 |  | 46,250 |  |  |  |  |  |  |  |  |
Unamortized grant date fair value |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance |  |  |  |  |  |  | $ 13.3 |  | $ 14.1 |  | $ 23.7 |  |  |  |  |  |  |  |  |
Issued |  |  |  |  |  |  | 15.7 |  | 9.9 |  | 6.7 |  |  |  |  |  |  |  |  |
Assumed forfeitures and cancelations |  |  |  |  |  |  | (0.2) |  | 0 |  | (0.1) |  |  |  |  |  |  |  |  |
Modified |  |  |  |  |  |  | 0 | [1] | 0 | [1] | (3.3) | [1] |  |  |  |  |  |  |  |
Expense recognized |  |  |  |  |  |  | (12.0) |  | (10.7) |  | (12.9) |  |  |  |  |  |  |  |  | (8.7)
Ending balance |  |  |  |  |  |  | $ 16.8 |  | $ 13.3 |  | $ 14.1 |  |  |  |  |  |  |  |  |
Number of installments in which the shares vest |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 |  |  |  |  |  |
Number of restricted shares installments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |
[1] | During the first quarter of 2007, White Mountains issued 15,000 restricted shares to the Company’s Chairman and CEO in connection with his hiring that would vest in the event of a change in control of the Company before January 20, 2012. During 2010, the Compensation Committee modified the vesting terms so that the 15,000 restricted shares time vest in three equal annual installments beginning on January 20, 2013. During 2012, the Compensation Committee accelerated the vesting date of the first installment from January 20, 2013 to December 31, 2012.

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph g(1) -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 FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph 64 -Subparagraph b -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 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=6793087&loc=d3e301413-122809 + Details
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- Definition As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph h -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. + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(e) -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. + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(d) -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. + Details
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- Definition An excess of the fair value of the modified award over the fair value of the award immediately before the modification. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph g(2) -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. + Details
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- Definition Number of share instruments newly issued under a share-based compensation plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph 65 -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 FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -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. + Details
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