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Significant Transactions (Dispositions) (Details) - USD ($) $ in Millions |  |  |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Sep. 28, 2017 | Oct. 05, 2016 | Jul. 21, 2016 | Apr. 18, 2016 | Sep. 30, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Significant Transactions |  |  |  |  |  |  |  |
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold |  |  |  |  |  | $ 1,131.0 | $ 2,646.2 | $ 24.0
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  | 557.0 | 415.1 | 18.2
Repayments of debt, principal |  |  |  |  |  | 365.0 | 404.6 | 76.1
Payments to acquire other investments |  |  |  |  |  | 84.1 | 38.5 | 73.9
OneBeacon |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold | $ 1,300.0 |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax | $ 554.6 |  |  |  |  | 554.5 | 0.0 | 0.0
Tranzact |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold |  | $ 1.2 | $ 221.3 |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  | 51.9 |  |  |  | 3.2 | 51.9 | 0.0
Repayments of debt, principal |  |  | 56.3 |  |  |  |  |
Discontinued operation, tax effect of discontinued operation |  | 30.2 |  |  |  |  |  |
Increase of book value |  | $ 82.1 | $ 82.1 |  |  |  |  |
Sirius Group |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold |  |  |  | $ 2,600.0 |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  | $ (4.0) |  | 363.2 |
Payments to acquire other investments |  |  |  | $ 161.8 |  |  |  |
Sale of Sirius Group |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  | (0.7) | 363.2 | 0.0
Recognition of benefit plan assets and obligations from sale of OneBeacon |  |  |  |  |  |  | 113.3 |
Discontinued Operations | OneBeacon |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  | 554.5 | 0.0 |
Discontinued operation, tax effect of discontinued operation |  |  |  |  |  | (5.7) | (12.5) | (12.9)
Recognition of benefit plan assets and obligations from sale of OneBeacon |  |  |  |  |  | 2.9 | 0.0 |
Discontinued Operations | Tranzact |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  | 3.2 | 51.9 |
Discontinued operation, tax effect of discontinued operation |  |  |  |  |  | 0.0 | (6.4) |
Recognition of benefit plan assets and obligations from sale of OneBeacon |  |  |  |  |  | 0.0 | 0.0 |
Discontinued Operations | Sirius Group |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  | (0.7) | 363.2 |
Discontinued operation, tax effect of discontinued operation |  |  |  |  |  | 0.0 | (3.1) | $ 27.1
Recognition of benefit plan assets and obligations from sale of OneBeacon |  |  |  |  |  | $ 0.0 | $ 113.3 |

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