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Significant Transactions (Dispositions) (Details) - USD ($) $ / shares in Units, $ in Millions |  | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Apr. 18, 2016 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2014 | Mar. 31, 2013 | Jun. 30, 2016 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Feb. 01, 2016 | Dec. 23, 2014
Significant Transactions |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  | $ 415.1 | $ 18.2 | $ (1.6) |  |
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax |  |  |  |  |  |  |  |  |  | 0.0 | 0.0 |  |
Total investments |  | $ 5,334.8 |  |  |  |  |  |  | 5,334.8 | 4,271.1 |  |  |
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold |  |  |  |  |  |  |  |  | 2,646.2 | 24.0 | 0.0 |  |
Payments to Acquire Other Investments |  |  |  |  |  |  |  |  | 39.5 | 78.0 | 114.7 |  |
Repayments of debt, principal |  |  |  |  |  |  |  |  | 404.6 | 76.1 | 65.2 |  |
Sales of common equity securities |  |  |  |  |  |  |  |  | $ 1,200.8 | 461.4 | $ 609.8 |  |
OneBeacon |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of quota share reinsurance agreement |  |  |  |  |  |  |  |  | 100.00% |  |  |  |
Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |  |  |  |  |  |
Sales of common equity securities |  |  |  |  | $ 658.0 |  |  |  |  |  |  |  |
Dividends Payable, Amount Per Share |  |  |  |  |  |  |  |  |  |  |  | $ 32.00 |
Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  | $ 363.2 |  |  |  |  | $ 363.2 |  |  |  |
Total investments |  |  |  |  |  |  |  |  |  | 686.2 |  |  |
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold | $ 2,600.0 |  |  | 2,600.0 |  |  |  |  |  |  |  |  |
Payments to Acquire Other Investments | $ 161.8 |  |  | $ 161.8 |  |  |  |  |  |  |  |  |
Sale of Sirius Group [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |  |  |  |  |  |
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax |  |  |  |  |  |  |  | $ 113.3 | 113.3 |  |  |  |
Tranzact |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  | $ 51.9 |  |  |  |  |  | $ 51.9 |  |  |  |
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold |  | $ 1.2 | 221.3 |  |  |  |  |  |  |  |  |  |
Repayments of debt, principal |  |  | 56.3 |  |  |  |  |  |  |  |  |  |
Discontinued Operation, Tax Effect of Discontinued Operation |  |  | 30.2 |  |  |  |  |  |  |  |  |  |
Increase of Book Value |  |  | $ 82.1 |  |  |  |  |  |  |  |  |  |
OneBeacon Runoff |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  |  |  |  |  |  | $ 101.0
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  | $ 36.1 |  |  |  |  |  |  |
Essentia | OneBeacon |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Disposition of Subsidiary |  |  |  |  |  |  | $ 23.0 |  |  |  |  |  |
Gain (Loss) on Disposition of Subsidiary, After Tax |  |  |  |  |  |  | $ 15.0 |  |  |  |  |  |
Gain (Loss) from Adjustments to Prior Period Extraordinary Items |  |  |  |  |  |  |  |  |  | (3.7) |  |  |
Symetra |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |  |  |  |  |  |
Sales of common equity securities |  |  |  |  | $ 658.0 |  |  |  |  |  |  |  |
Symetra | Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |  |  |  |  |  |
Total investments |  |  |  |  |  |  |  |  |  | 528.6 |  |  |
Fair Value [Member] | Other long-term investments | OneBeacon Runoff |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  |  |  |  |  |  | $ 64.9
Crop Business |  |  |  |  |  |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds related from termination of agreement |  |  |  |  |  |  |  |  |  | $ 3.0 |  |  |
Percentage of quota share reinsurance agreement |  |  |  |  |  |  |  |  |  | 100.00% |  |  |

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