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Discontinued Operations (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Basic and diluted earnings per share numerators (in millions): |  |  |  |  |  |  |  |  |  |  |
Allocation of income for participating unvested restricted common shares |  |  |  |  |  |  |  |  | $ (2.3) | $ (0.7) | $ (2.6)
Net income attributable to White Mountains’s common shareholders | $ 267.9 | $ (58.9) | $ 4.3 | $ 84.3 | $ 69.9 | $ 51.3 | $ 95.5 | $ 95.5 | $ 297.6 | $ 312.2 | $ 321.6
Basic earnings per share denominators (in thousands): |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period |  |  |  |  |  |  |  |  | 5,879,200 | 6,104,900 | 6,200,400
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (68,000) | (78,900) | (91,400)
Basic earnings per share denominator |  |  |  |  |  |  |  |  | 5,811,200 | 6,026,000 | 6,109,000
Diluted earnings per share denominator (in thousands): |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period |  |  |  |  |  |  |  |  | 5,879,200 | 6,104,900 | 6,200,400
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (68,000) | (78,900) | (91,400)
Average outstanding dilutive options to acquire common shares |  |  |  |  |  |  |  |  | 0 | 0 | 0
Diluted earnings per share denominator |  |  |  |  |  |  |  |  | 5,811,200 | 6,026,000 | 6,109,000
Incremental common shares attributable to share-based payment arrangements |  |  |  |  |  |  |  |  | 125,000 | 125,000 | 125,000
Discontinued Operations |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total income from discontinued operations |  |  |  |  |  |  |  |  | $ 98.8 | $ 258.6 | $ 146.5
Basic and diluted earnings per share numerators (in millions): |  |  |  |  |  |  |  |  |  |  |
Allocation of income for participating unvested restricted common shares |  |  |  |  |  |  |  |  | (1.1) | 3.3 | (2.2)
Net income attributable to White Mountains’s common shareholders |  |  |  |  |  |  |  |  | $ 97.7 | $ 255.3 | $ 144.3
Basic earnings per share denominators (in thousands): |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period |  |  |  |  |  |  |  |  | 5,879,200 | 6,104,900 | 6,200,400
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (68,000) | (78,900) | (91,400)
Basic earnings per share denominator |  |  |  |  |  |  |  |  | 5,811,200 | 6,026,000 | 6,109,000
Diluted earnings per share denominator (in thousands): |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period |  |  |  |  |  |  |  |  | 5,879,200 | 6,104,900 | 6,200,400
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (68,000) | (78,900) | (91,400)
Average outstanding dilutive options to acquire common shares |  |  |  |  |  |  |  |  | 0 | 0 | 0
Diluted earnings per share denominator |  |  |  |  |  |  |  |  | 5,811,200 | 6,026,000 | 6,109,000
Basic and diluted earnings per share (in dollars): |  |  |  |  |  |  |  |  | $ 16.80 | $ 42.36 | $ 23.63
Incremental common shares attributable to share-based payment arrangements |  |  |  |  |  |  |  |  | 125,000 | 125,000 | 125,000

X
- Definition The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period. + References No definition available. + Details
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- Definition Amount after tax of income (loss) from a discontinued operation attributable to the parent. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 5C -Subparagraph (b)(2) -URI http://asc.fasb.org/extlink&oid=51717284&loc=SL51721675-107760 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3B -URI http://asc.fasb.org/extlink&oid=51716988&loc=SL51721525-107759 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3A -URI http://asc.fasb.org/extlink&oid=51716988&loc=SL51721523-107759 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=51716988&loc=d3e957-107759 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=51716988&loc=d3e1012-107759 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=51717284&loc=d3e1361-107760 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 5B -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=51717284&loc=SL51721673-107760 Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 19 -URI http://asc.fasb.org/extlink&oid=51664549&loc=SL4569616-111683 + Details
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- Definition Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 28A -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1500-109256 + Details
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- Definition Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1448-109256 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Weighted-Average Number of Common Shares Outstanding -URI http://asc.fasb.org/extlink&oid=6528421 + Details
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- Definition Number of shares of restricted stock determined by relating the portion of time within a reporting period that restricted shares have been outstanding to the total time in that period. Restricted shares are subject to sales, contractual, regulatory or other restrictions that prevent or inhibit the holder from freely disposing of them before the restriction ends. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e2646-109256 + Details
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