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Held for Sale and Discontinued Operations (Loss from Discontinued Operations) (Details) - USD ($) $ in Millions |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Sep. 28, 2017 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Sep. 28, 2017 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Revenues |  |  |  |  |  |  |  |  |  |  |  |  |
Earned insurance premiums |  |  |  |  |  |  |  |  |  |  | $ 16.3 | $ 13.9 | $ 10.4
Net investment income |  |  |  |  |  |  |  |  |  |  | 79.7 | 59.0 | 56.0
Other revenue |  |  |  |  |  |  |  |  |  |  | 47.6 | 15.3 | 7.1
Net realized and unrealized investment gains (losses) |  |  |  |  |  |  |  |  |  |  | 433.2 | (108.3) | 133.3
Revenues |  | $ 175.3 | $ 155.5 | $ 129.0 | $ 433.6 | $ 6.0 | $ 198.7 | $ 122.3 | $ 42.1 |  | 893.4 | 369.1 | 373.8
Expenses |  |  |  |  |  |  |  |  |  |  |  |  |
Loss and loss adjustment expenses |  |  |  |  |  |  |  |  |  |  |  |  | 1.1
Other underwriting expenses |  |  |  |  |  |  |  |  |  |  | 0.4 | 0.4 | 0.4
General and administrative expenses |  |  |  |  |  |  |  |  |  |  | 327.8 | 233.5 | 208.0
Interest expense on debt |  |  |  |  |  |  |  |  |  |  | 17.6 | 9.5 | 2.3
Total expenses |  | 118.8 | 104.4 | 113.2 | 151.9 | 150.1 | 154.4 | 134.7 | 108.1 |  | 488.3 | 547.3 | 366.0
Net income from discontinued operations, net of tax |  | $ (0.8) | $ 0.9 | $ 0.0 | $ 0.7 | $ 0.0 | $ (17.3) | $ 0.0 | $ 0.1 |  | 0.0 | 0.0 | 20.5
Gain (loss) on sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  | 0.8 | (17.2) | 557.0
Total income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  |  | 0.8 | (17.2) | 577.5
Comprehensive income from discontinued operations |  |  |  |  |  |  |  |  |  |  | 413.1 | (146.0) | 630.7
Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |  |
Expenses |  |  |  |  |  |  |  |  |  |  |  |  |
Total income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  |  | 0.8 | (17.2) | 577.5
Recognition of foreign currency translation from sale of OneBeacon, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
OneBeacon |  |  |  |  |  |  |  |  |  |  |  |  |
Expenses |  |  |  |  |  |  |  |  |  |  |  |  |
Net income from discontinued operations, net of tax |  |  |  |  |  |  |  |  |  | $ 20.5 |  |  |
Gain (loss) on sale of other discontinued operations, net of tax | $ 554.6 |  |  |  |  |  |  |  |  |  |  |  |
OneBeacon | Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  |  |  |
Earned insurance premiums |  |  |  |  |  |  |  |  |  |  |  |  | 807.6
Net investment income |  |  |  |  |  |  |  |  |  |  |  |  | 39.7
Other revenue |  |  |  |  |  |  |  |  |  |  |  |  | 7.7
Net realized and unrealized investment gains (losses) |  |  |  |  |  |  |  |  |  |  |  |  | 38.8
Revenues |  |  |  |  |  |  |  |  |  |  |  |  | 893.8
Expenses |  |  |  |  |  |  |  |  |  |  |  |  |
Loss and loss adjustment expenses |  |  |  |  |  |  |  |  |  |  |  |  | 546.0
Insurance and reinsurance acquisition expenses |  |  |  |  |  |  |  |  |  |  |  |  | 145.6
Other underwriting expenses |  |  |  |  |  |  |  |  |  |  |  |  | 156.2
General and administrative expenses |  |  |  |  |  |  |  |  |  |  |  |  | 21.2
Interest expense on debt |  |  |  |  |  |  |  |  |  |  |  |  | 10.0
Total expenses |  |  |  |  |  |  |  |  |  |  |  |  | 879.0
Net income from discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  | 14.8
Income tax benefit |  |  |  |  |  |  |  |  |  |  |  |  | 5.7
Net income from discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 20.5
Gain (loss) on sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 554.5
Total income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  | 575.0
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 0.3
Recognition of foreign currency translation from sale of OneBeacon, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 2.9
Comprehensive income from discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  | 578.2
Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |
Expenses |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) on sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  | (17.3) |
Sirius Group | Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  |  |  |
Earned insurance premiums |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Net investment income |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Other revenue |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Net realized and unrealized investment gains (losses) |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Revenues |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Expenses |  |  |  |  |  |  |  |  |  |  |  |  |
Loss and loss adjustment expenses |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Insurance and reinsurance acquisition expenses |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Other underwriting expenses |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
General and administrative expenses |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Interest expense on debt |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Total expenses |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Net income from discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Income tax benefit |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Net income from discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Gain (loss) on sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  | $ 0.8 | (17.2) | (0.7)
Total income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  | (0.7)
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Comprehensive income from discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  | (0.7)
Tranzact |  |  |  |  |  |  |  |  |  |  |  |  |
Expenses |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) on sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  | $ 3.2 | 3.2
Tranzact | Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  |  |  |
Earned insurance premiums |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Net investment income |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Other revenue |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Net realized and unrealized investment gains (losses) |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Revenues |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Expenses |  |  |  |  |  |  |  |  |  |  |  |  |
Loss and loss adjustment expenses |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Insurance and reinsurance acquisition expenses |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Other underwriting expenses |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
General and administrative expenses |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Interest expense on debt |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Total expenses |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Net income from discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Income tax benefit |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Net income from discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Gain (loss) on sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 3.2
Total income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  | 3.2
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 0.0
Comprehensive income from discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  | 3.2
Total | Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  |  |  |  |  |
Earned insurance premiums |  |  |  |  |  |  |  |  |  |  |  |  | 807.6
Net investment income |  |  |  |  |  |  |  |  |  |  |  |  | 39.7
Other revenue |  |  |  |  |  |  |  |  |  |  |  |  | 7.7
Net realized and unrealized investment gains (losses) |  |  |  |  |  |  |  |  |  |  |  |  | 38.8
Revenues |  |  |  |  |  |  |  |  |  |  |  |  | 893.8
Expenses |  |  |  |  |  |  |  |  |  |  |  |  |
Loss and loss adjustment expenses |  |  |  |  |  |  |  |  |  |  |  |  | 546.0
Insurance and reinsurance acquisition expenses |  |  |  |  |  |  |  |  |  |  |  |  | 145.6
Other underwriting expenses |  |  |  |  |  |  |  |  |  |  |  |  | 156.2
General and administrative expenses |  |  |  |  |  |  |  |  |  |  |  |  | 21.2
Interest expense on debt |  |  |  |  |  |  |  |  |  |  |  |  | 10.0
Total expenses |  |  |  |  |  |  |  |  |  |  |  |  | 879.0
Net income from discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  | 14.8
Income tax benefit |  |  |  |  |  |  |  |  |  |  |  |  | 5.7
Net income from discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 20.5
Gain (loss) on sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 557.0
Change in foreign currency translation and other comprehensive income from discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 0.3
Recognition of foreign currency translation from sale of OneBeacon, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 2.9
Comprehensive income from discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  | $ 580.7

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