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Significant Transactions (Details) - USD ($) |  |  |  |  |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |
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Sep. 28, 2017 | Aug. 01, 2017 | Oct. 05, 2016 | Aug. 04, 2016 | Jul. 21, 2016 | Apr. 18, 2016 | Feb. 01, 2016 | Sep. 30, 2017 | Mar. 31, 2017 | Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016 | Jun. 30, 2017 | Jun. 30, 2016 | Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including 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 |  |  |  |
Assets |  |  |  |  |  |  |  | 3,637,100,000 |  |  |  | 3,637,100,000 |  | $ 6,520,200,000 |  |  |
Goodwill |  |  |  |  |  |  |  | 25,900,000 |  | 25,900,000 |  | 25,900,000 | 25,900,000 | 25,900,000 | $ 25,900,000 | $ 18,300,000 | $ 18,600,000
Liabilities |  |  |  |  |  |  |  | $ 299,900,000 |  |  |  | 299,900,000 |  | 2,804,200,000 |  |  |
Repayments of debt |  |  |  |  |  |  |  |  |  |  |  | 364,600,000 | 401,800,000 |  |  |  |
Payments to acquire other investments |  |  |  |  |  |  |  |  |  |  |  | 67,700,000 | 36,600,000 |  |  |  |
Sales of common equity securities |  |  |  |  |  |  |  |  |  |  |  | $ 407,800,000 | 683,000,000 |  |  |  |
Buzzmove |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  | 70.90% |  |  |  | 77.10% |  |  |  | 77.10% |  |  |  |  |
Assets |  |  |  | $ 11,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  | 7,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible Assets, Net (Excluding Goodwill) |  |  |  | 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Liabilities |  |  |  | 100,000 |  |  |  |  |  |  |  |  |  |  |  |  |
United Kingdom, Pounds | Buzzmove |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition of subsidiary |  |  |  | 6,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |
United States of America, Dollars | Buzzmove |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition of subsidiary |  |  |  | $ 8,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |
OneBeacon |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including 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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tranzact |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including 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 |  |  |
Repayments of debt |  |  |  |  | 56,300,000 |  |  |  |  |  |  |  |  |  |  |  |
Discontinued operation, tax effect of discontinued operation |  |  | 30,200,000 |  |  |  |  |  |  | 15,900,000 |  |  | 13,800,000 | 30,200,000 |  |  |
Increase of book value |  |  | $ 82,100,000 |  | $ 82,100,000 |  |  |  |  |  |  |  |  |  |  |  |
Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including 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 |  |  |  |  |  |  |  |  |  |  |
Sale of Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary |  |  |  |  |  |  |  |  |  |  |  |  |  | 113,300,000 |  |  |
Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sales of common equity securities |  |  |  |  |  |  | $ 658,000,000 |  |  |  |  |  |  |  |  |  |
Dividends payable, amount per share |  |  |  |  |  |  | $ 32.00 |  |  |  |  |  |  |  |  |  |
Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sales of common equity securities |  |  |  |  |  |  | $ 658,000,000 |  |  |  |  |  |  |  |  |  |
Common stock, dividends, per share, cash paid |  |  |  |  |  |  | $ 0.50 |  |  |  | $ 0.50 |  |  |  |  |  |
Preferred Stock | Buzzmove |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition of subsidiary |  | $ 5,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock issued during period, shares, acquisitions |  | 37,409 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock | United Kingdom, Pounds | Buzzmove |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition of subsidiary |  | $ 4,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock | Buzzmove |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition of subsidiary |  | $ 700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock issued during period, shares, acquisitions |  | 5,808 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock | United Kingdom, Pounds | Buzzmove |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition of subsidiary |  | $ 500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued Operations | OneBeacon |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued operation, tax effect of discontinued operation |  |  |  |  |  |  |  | $ 4,100,000 |  | (10,400,000) |  | $ 5,700,000 | (300,000) |  |  |  |
Discontinued Operations | Tranzact |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discontinued operation, tax effect of discontinued operation |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 30.2 |  |  |
Discontinued Operations | Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain from sale of other discontinued operations, net of tax |  |  |  |  |  |  |  | (800,000) |  |  |  | $ 1,000,000 |  |  |  |  |
Discontinued operation, tax effect of discontinued operation |  |  |  |  |  |  |  | $ 0 |  | $ 0 |  |  | $ 3,100,000 |  |  |  |

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- Definition Increase of Book Value + References No definition available. + Details
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- Definition The cash inflow associated with the sale of equity securities classified as trading. + References No definition available. + Details
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