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Discontinued Operations - (Details) - USD ($) |  |  |  |  | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 28, 2017 | Oct. 05, 2016 | Jul. 21, 2016 | Apr. 18, 2016 | Sep. 30, 2017 | Mar. 31, 2017 | Sep. 30, 2016 | Jun. 30, 2016 | Jun. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016 | Dec. 31, 2015
Discontinued Operations [Line Items} |  |  |  |  |  |  |  |  |  |  |  |  |
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold |  |  |  |  |  |  |  |  |  | $ 1,131,000,000 | $ 2,657,200,000 |  |
Gain from sale of other discontinued operations, net of tax |  |  |  |  | $ 554,300,000 |  | $ 47,900,000 |  |  | 552,700,000 | 414,500,000 |  |
Net (loss) income from discontinued operations |  |  |  |  | (15,200,000) |  | 36,500,000 |  |  | 20,500,000 | 100,900,000 |  |
Income tax expense (benefit) |  |  |  |  | (4,000,000) |  | (17,100,000) |  |  | (5,300,000) | (22,700,000) |  |
Payments to acquire other investments |  |  |  |  |  |  |  |  |  | 67,700,000 | 36,600,000 |  |
Investments |  |  |  |  | 3,280,500,000 |  |  |  |  | 3,280,500,000 |  | $ 2,714,400,000 |
Net realized and unrealized investment gains |  |  |  |  | 32,500,000 |  | 10,900,000 |  |  | 102,500,000 | 27,200,000 |  |
Net income attributable to White Mountains’s common shareholders |  |  |  |  | (539,100,000) |  | (84,400,000) |  |  | (573,200,000) | (515,400,000) |  |
Net change in foreign currency translation and benefit plan assets and obligations |  |  |  |  | 0 |  | 200,000 |  |  | 200,000 | (400,000) |  |
OneBeacon |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operations [Line Items} |  |  |  |  |  |  |  |  |  |  |  |  |
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold | $ 1,300,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Gain from sale of other discontinued operations, net of tax | $ 554,600,000 |  |  |  |  |  |  |  |  |  |  |  |
Net (loss) income from discontinued operations |  |  |  |  | (15,200,000) |  | 22,500,000 |  |  | 20,500,000 | 93,800,000 |  |
Tranzact |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operations [Line Items} |  |  |  |  |  |  |  |  |  |  |  |  |
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold |  | $ 1,200,000 | $ 221,300,000 |  |  |  |  |  |  |  |  |  |
Gain from sale of other discontinued operations, net of tax |  | 51,900,000 |  |  |  | $ (1,000,000) |  |  |  |  |  | 51,900,000 |
Net (loss) income from discontinued operations |  |  |  |  |  |  | 14,000,000 |  |  |  | 13,500,000 |  |
Discontinued operation, tax effect of discontinued operation |  | $ 30,200,000 |  |  |  |  | 15,900,000 |  |  |  | 13,800,000 | 30,200,000 |
Net realized and unrealized investment gains |  |  |  |  |  |  | 0 |  |  |  | 0 |  |
Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operations [Line Items} |  |  |  |  |  |  |  |  |  |  |  |  |
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold |  |  |  | $ 2,600,000,000 |  |  |  |  |  |  |  |  |
Gain from sale of other discontinued operations, net of tax |  |  |  |  |  |  | (4,000,000) |  |  |  |  | 363,200,000 |
Payments to acquire other investments |  |  |  | $ 161,800,000 |  |  |  |  |  |  |  |  |
Investments |  |  |  |  |  |  |  |  |  |  |  |  | $ 686,200,000
Net realized and unrealized investment gains |  |  |  |  |  |  |  | $ 400,000 | $ 3,700,000 |  |  |  |
Sale of Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operations [Line Items} |  |  |  |  |  |  |  |  |  |  |  |  |
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary |  |  |  |  |  |  |  |  |  |  |  | 113,300,000 |
Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operations [Line Items} |  |  |  |  |  |  |  |  |  |  |  |  |
Net income attributable to White Mountains’s common shareholders |  |  |  |  | 539,100,000 |  | 84,400,000 |  |  | 573,200,000 | 515,400,000 |  |
Net change in foreign currency translation and benefit plan assets and obligations |  |  |  |  |  |  |  |  |  |  | 32,500,000 |  |
Discontinued Operations | OneBeacon |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operations [Line Items} |  |  |  |  |  |  |  |  |  |  |  |  |
Net (loss) income from discontinued operations |  |  |  |  | (15,200,000) |  | 22,500,000 |  |  | 20,500,000 | 93,800,000 |  |
Discontinued operation, tax effect of discontinued operation |  |  |  |  | 4,100,000 |  | (10,400,000) |  |  | 5,700,000 | (300,000) |  |
Net realized and unrealized investment gains |  |  |  |  | 11,500,000 |  | 15,500,000 |  |  | 38,800,000 | 56,800,000 |  |
Net income attributable to White Mountains’s common shareholders |  |  |  |  | 539,300,000 |  | 22,500,000 |  |  | 575,000,000 | 93,800,000 |  |
Net change in foreign currency translation and benefit plan assets and obligations |  |  |  |  | 100,000 |  | (300,000) |  |  | 300,000 | 500,000 |  |
Discontinued Operations | Tranzact |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operations [Line Items} |  |  |  |  |  |  |  |  |  |  |  |  |
Net (loss) income from discontinued operations |  |  |  |  |  |  | 2,100,000 |  |  |  | 2,600,000 |  |
Discontinued operation, tax effect of discontinued operation |  |  |  |  |  |  |  |  |  |  |  | 30.2 |
Income tax expense (benefit) |  |  |  |  |  |  |  |  |  |  |  | $ 16.1 |
Net income attributable to White Mountains’s common shareholders |  |  |  |  |  |  | 65,900,000 |  |  | (1,000,000) | 65,400,000 |  |
Discontinued Operations | Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operations [Line Items} |  |  |  |  |  |  |  |  |  |  |  |  |
Gain from sale of other discontinued operations, net of tax |  |  |  |  | (800,000) |  |  |  |  | 1,000,000 |  |  |
Net (loss) income from discontinued operations |  |  |  |  | 0 |  | 0 |  |  |  | (6,400,000) |  |
Discontinued operation, tax effect of discontinued operation |  |  |  |  | 0 |  | 0 |  |  |  | 3,100,000 |  |
Net realized and unrealized investment gains |  |  |  |  | 0 |  | 0 |  |  |  | (1,500,000) |  |
Net income attributable to White Mountains’s common shareholders |  |  |  |  | $ (200,000) |  | (4,000,000) |  |  | $ (800,000) | 356,200,000 |  |
Other comprehensive income |  |  |  |  |  |  |  |  |  |  | 145,300,000 |  |
Net change in foreign currency translation and benefit plan assets and obligations |  |  |  |  |  |  | 0 |  |  |  | 32,000,000 |  |
Discontinued Operations | Sale of Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operations [Line Items} |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in foreign currency translation and benefit plan assets and obligations |  |  |  |  |  |  | 0 |  |  |  | 113,300,000 |  |
Continuing Operations |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operations [Line Items} |  |  |  |  |  |  |  |  |  |  |  |  |
Income tax expense (benefit) |  |  |  |  |  |  |  |  |  |  | $ (14,000,000) |  |
Continuing Operations | Tranzact |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operations [Line Items} |  |  |  |  |  |  |  |  |  |  |  |  |
Income tax expense (benefit) |  |  |  |  |  |  | $ 14,000,000 |  |  |  |  |  |
Symetra | Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operations [Line Items} |  |  |  |  |  |  |  |  |  |  |  |  |
Investments |  |  |  |  |  |  |  |  |  |  |  |  | $ 528,600,000
Intact Financial Corporation [Member] | OneBeacon |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operations [Line Items} |  |  |  |  |  |  |  |  |  |  |  |  |
Sale of Stock, Price Per Share | $ 18.10 |  |  |  |  |  |  |  |  |  |  |  |

X
- Definition Amount after tax of gain (loss) not previously recognized resulting from the disposal of a discontinued operation. + References Reference 1: 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=64827811&loc=d3e1361-107760 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=65878427&loc=SL51721525-107759 Reference 3: 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=64827811&loc=SL51721675-107760 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=65878427&loc=d3e957-107759 + Details
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- Definition Amount of tax expense (benefit) related to a discontinued operation. Includes, but is not limited to, tax expense (benefit) related to income (loss) from operations during the phase-out period, tax expense (benefit) related to gain (loss) on disposal, tax expense (benefit) related to gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and tax expense (benefit) related to adjustments of a prior period gain (loss) on disposal. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32672-109319 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=65878427&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=65878427&loc=SL51721523-107759 Reference 4: 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=64827811&loc=d3e1361-107760 Reference 5: 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=64827811&loc=SL51721673-107760 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=65896914&loc=d3e38679-109324 Reference 7: 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=64827811&loc=SL51721675-107760 Reference 8: 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=65878427&loc=d3e1012-107759 + Details
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- Definition This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (c)-(e) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c),9(a)) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27405-111563 + Details
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- Definition The cash inflow associated with the deconsolidation of a previously consolidated subsidiary or sale of an entity that is related to it but not strictly controlled. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3179-108585 + Details
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