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Discontinued Operations (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions |  |  |  |  | 3 Months Ended | 12 Months Ended
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Sep. 28, 2017 | Oct. 05, 2016 | Jul. 21, 2016 | Apr. 18, 2016 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 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 |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,131.0 | $ 2,646.2 | $ 24.0
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 557.0 | 415.1 | 18.2
Net (loss) income from discontinued operations, net of tax |  |  |  |  | $ 4.3 | $ 539.1 | $ 2.8 | $ 31.3 | $ 8.0 | $ 84.4 | $ 383.6 | $ 47.4 | 20.5 | 108.3 | 116.9
Income Tax (expense) benefit |  |  |  |  | 2.5 | $ 4.0 | $ 1.0 | $ 0.3 | 10.2 | 17.1 | $ 4.0 | $ 1.6 | 7.8 | 32.9 | (12.7)
Payments to acquire other investments |  |  |  |  |  |  |  |  |  |  |  |  | 84.1 | 38.5 | 73.9
Total investments |  |  |  |  | $ 3,380.7 |  |  |  | $ 2,714.4 |  |  |  | 3,380.7 | 2,714.4 |
Net realized and unrealized investment gains |  |  |  |  |  |  |  |  |  |  |  |  | 133.3 | (27.4) | 260.5
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 | $ 1,300.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax | $ 554.6 |  |  |  |  |  |  |  |  |  |  |  | 554.5 | 0.0 | 0.0
Net (loss) income from discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 20.5 | 108.6 | 38.2
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 |  | $ 1.2 | $ 221.3 |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  | 51.9 |  |  |  |  |  |  |  |  |  |  | 3.2 | 51.9 | 0.0
Discontinued operation, tax effect of discontinued operation |  | $ 30.2 |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  | $ 2,600.0 |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  | $ (4.0) |  |  |  | 363.2 |
Payments to acquire other investments |  |  |  | $ 161.8 |  |  |  |  |  |  |  |  |  |  |
Total investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 686.2
Net realized and unrealized investment gains |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.7 | 205.0
Sirius Group | Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 528.6
Net realized and unrealized investment gains |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 200.8
Sale of Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | (0.7) | 363.2 | 0.0
Recognition of other comprehensive income from sale of subsidiary |  |  |  |  |  |  |  |  |  |  |  |  |  | 113.3 |
Esurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 | 0.0 | 17.9
Discontinued Operations | OneBeacon |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 554.5 | 0.0 |
Net (loss) income from discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 20.5 | 108.6 | 38.2
Discontinued operation, tax effect of discontinued operation |  |  |  |  |  |  |  |  |  |  |  |  | (5.7) | (12.5) | (12.9)
Recognition of other comprehensive income from sale of subsidiary |  |  |  |  |  |  |  |  |  |  |  |  | 2.9 | 0.0 |
Net realized and unrealized investment gains |  |  |  |  |  |  |  |  |  |  |  |  | 38.8 | 37.7 | (35.1)
Net change in pension liability and other accumulated comprehensive items |  |  |  |  |  |  |  |  |  |  |  |  | 0.3 | 1.0 | 0.0
Discontinued Operations | Tranzact |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 3.2 | 51.9 |
Net (loss) income from discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 | 6.1 | (1.9)
Income Tax (expense) benefit |  |  |  |  |  |  |  |  |  |  |  |  |  | 21.4 |
Tax effect of discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  |  | 30.2 |
Discontinued operation, tax effect of discontinued operation |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 | (6.4) |
Recognition of other comprehensive income from sale of subsidiary |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 | 0.0 |
Net realized and unrealized investment gains |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 | 0.0 |
Net change in pension liability and other accumulated comprehensive items |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 | 0.0 |
Discontinued Operations | Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | (0.7) | 363.2 |
Net (loss) income from discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 | (6.4) |
Discontinued operation, tax effect of discontinued operation |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 | (3.1) | 27.1
Recognition of other comprehensive income from sale of subsidiary |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 | 113.3 |
Net realized and unrealized investment gains |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 | (1.5) | 15.1
Net change in pension liability and other accumulated comprehensive items |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | 32.0 | (65.0)
Discontinued Operations | Sale of Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net (loss) income from discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  |  | (4.3) | 81.1
Net change in pension liability and other accumulated comprehensive items |  |  |  |  |  |  |  |  |  |  |  |  |  | 32.0 |
Discontinued Operations | Esurance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (loss) from sale of other discontinued operations, net of tax |  |  |  |  |  |  |  |  |  |  |  |  |  | 28.3 | $ 17.9
OneBeacon | Intact Financial Corporation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sale of stock, price per share | $ 18.10 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net gain (loss) from sale of discontinued operations | 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 |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 8.8 |

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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=84230637&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=79507043&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 Includes changes, net of tax, in the net (gain) loss, prior service cost (credit), and transition assets (obligations), as well as minimum pension liability if still remaining, included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost. Also includes change, net of tax, in accumulated gains and losses from derivative instruments (interest rate swaps) designated and qualifying as the effective portion of cash flow hedges. + References No definition available. + Details
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