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CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions | Total | Common shares and paid-in surplus | Retained earnings | AOCL, after-tax | Parent | Non-controlling Interests | Total Equity includes Noncontrolling Interest [Member] | Foreign Currency Gain (Loss) Common shares and paid-in surplus | Foreign Currency Gain (Loss) Retained earnings | Foreign Currency Gain (Loss) AOCL, after-tax | Foreign Currency Gain (Loss) Parent | Foreign Currency Gain (Loss) Non-controlling Interests | Foreign Currency Gain (Loss) Total Equity includes Noncontrolling Interest [Member] | Pension liability and other accumulated comprehensive items Common shares and paid-in surplus | Pension liability and other accumulated comprehensive items Retained earnings | Pension liability and other accumulated comprehensive items AOCL, after-tax | Pension liability and other accumulated comprehensive items Parent | Pension liability and other accumulated comprehensive items Non-controlling Interests | Pension liability and other accumulated comprehensive items Total Equity includes Noncontrolling Interest [Member] | Recognition of foreign currency translation and other accumulated comprehensive items [Member] Common shares and paid-in surplus | Recognition of foreign currency translation and other accumulated comprehensive items [Member] Retained earnings | Recognition of foreign currency translation and other accumulated comprehensive items [Member] Non-controlling Interests | Recognition of foreign currency translation and other accumulated comprehensive items [Member] Total Equity includes Noncontrolling Interest [Member] | Prospector Turtle Fund Common shares and paid-in surplus | Prospector Turtle Fund Retained earnings | Prospector Turtle Fund AOCL, after-tax | Prospector Turtle Fund Parent | Prospector Turtle Fund Non-controlling Interests | Prospector Turtle Fund Total Equity includes Noncontrolling Interest [Member] | Buzzmove [Member] Common shares and paid-in surplus | Buzzmove [Member] Retained earnings | Buzzmove [Member] AOCL, after-tax | Buzzmove [Member] Parent | Buzzmove [Member] Non-controlling Interests | Tranzact Non-controlling Interests | Sirius Group Non-controlling Interests | Sirius Group Total Equity includes Noncontrolling Interest [Member] | Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] AOCL, after-tax | Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Parent | Sale of Sirius Group [Member] | Sale of Sirius Group [Member] Common shares and paid-in surplus | Sale of Sirius Group [Member] Retained earnings | Sale of Sirius Group [Member] AOCL, after-tax | Sale of Sirius Group [Member] Parent | Tranzact Non-controlling Interests | Tranzact Total Equity includes Noncontrolling Interest [Member]
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Beginning balance at Dec. 31, 2013 |  | $ 1,051.1 | $ 2,801.9 | $ 52.1 | $ 3,905.1 | $ 491.7 | $ 4,396.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income | $ 290.0 | 0.0 | 312.2 | 0.0 | 312.2 | (22.2) | 290.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax | 75.3 | 0.0 | 0.0 | 75.3 | 75.3 | 0.0 | 75.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss), after-tax |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ (8.7) | $ (8.7) | $ (3.0) | $ (11.7) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OCI, Change in foreign currency transaction and translation adjustment, net of tax | (10.7) |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ (168.2) | $ (168.2) | $ (0.3) | $ (168.5) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (169.5) |  |  |  |  |  |
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax | 0.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Comprehensive income | 207.3 | 0.0 | 312.2 | (101.6) | 210.6 | (25.5) | 185.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  | 0.0 | (6.2) | 0.0 | (6.2) | 0.0 | (6.2) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends/distributions to non-controlling interests |  | 0.0 | 0.0 | 0.0 | 0.0 | (40.7) | (40.7) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuances of common shares |  | 4.8 | 0.0 | 0.0 | 4.8 | 0.0 | 4.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares | (134.5) | (37.1) | (97.4) | 0.0 | (134.5) | 0.0 | (134.5) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest, Increase from Subsidiary Equity Issuance |  | 0.0 | 0.0 | 0.0 | 0.0 | 8.3 | 8.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest, Increase from Business Combination |  |  |  |  |  |  | 122.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 122.1 |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests |  |  |  |  |  |  | (14.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (14.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization of restricted share and option awards |  | 15.9 | 0.0 | 0.0 | 15.9 | 0.8 | 16.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending balance at Dec. 31, 2014 |  | 1,034.7 | 3,010.5 | (49.5) | 3,995.7 | 542.7 | 4,538.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income | 279.5 | 0.0 | 297.6 | 0.0 | 297.6 | (18.1) | 279.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax | (34.9) | 0.0 | 0.0 | (34.9) | (34.9) | 0.0 | (34.9) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss), after-tax |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | 0.3 | 0.3 | 0.0 | 0.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OCI, Change in foreign currency transaction and translation adjustment, net of tax | (0.5) |  |  |  |  |  |  | 0.0 | 0.0 | (65.8) | (65.8) | 0.0 | (65.8) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (65.0) |  |  |  |  |  |
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax | 0.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Comprehensive income | 197.2 | 0.0 | 297.6 | (100.4) | 197.2 | (18.1) | 179.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  | 0.0 | (6.0) | 0.0 | (6.0) | 0.0 | (6.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends/distributions to non-controlling interests |  | 0.0 | 0.0 | 0.0 | 0.0 | (51.1) | (51.1) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuances of common shares |  | 0.9 | 0.0 | 0.0 | 0.9 | 0.0 | 0.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares | (284.2) | (67.0) | (217.2) | 0.0 | (284.2) | 0.0 | (284.2) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest, Increase from Subsidiary Equity Issuance |  | 0.0 | 0.0 | 0.0 | 0.0 | 17.5 | 17.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests |  | (5.3) | 0.0 | 0.0 | (5.3) | (2.7) | (8.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ (31.5) | $ (31.5) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization of restricted share and option awards |  | 14.9 | 0.0 | 0.0 | 14.9 | (2.0) | 12.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending balance at Dec. 31, 2015 | 3,913.2 | 978.2 | 3,084.9 | (149.9) | 3,913.2 | 454.8 | 4,368.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income | 419.8 | 0.0 | 412.5 | 0.0 | 412.5 | 7.3 | 419.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax | 0.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss), after-tax |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.6 | $ 0.6 | $ 0.3 | $ 0.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
OCI, Change in foreign currency transaction and translation adjustment, net of tax | 0.3 |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 31.4 | $ 31.4 | $ 0.0 | $ 31.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 32.0 |  |  |  |  |  |
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 113.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 113.3 | $ 113.3 | $ 113.3 |  |  |  |  |  |
Comprehensive income | 558.1 | 0.0 | 412.5 | 145.3 | 557.8 | 7.6 | 565.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  | 0.0 | (5.4) | 0.0 | (5.4) | 0.0 | (5.4) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends/distributions to non-controlling interests |  | 0.0 | 0.0 | 0.0 | 0.0 | (22.7) | (22.7) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuances of common shares |  | 9.1 | 0.0 | 0.0 | 9.1 | 0.0 | 9.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares | (887.2) | (192.4) | (694.8) | 0.0 | (887.2) | 0.0 | (887.2) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest, Decrease from Deconsolidation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (250.0) | $ (250.0) |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ (78.4) | $ (78.4)
Noncontrolling Interest, Increase from Subsidiary Equity Issuance |  |  |  |  |  | 27.3 | 27.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest, Increase from Business Combination |  |  |  |  |  |  | 3.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ 3.3 |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests |  | (2.7) | 0.0 | 0.0 | (2.7) | (8.8) | (11.5) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization of restricted share and option awards |  | 18.5 | 0.0 | 0.0 | 18.5 | 0.8 | 19.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending balance at Dec. 31, 2016 | $ 3,603.3 | $ 810.7 | $ 2,797.2 | $ (4.6) | $ 3,603.3 | $ 133.9 | $ 3,737.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Decrease in noncontrolling interest balance from payment of dividends or other distributions by the non-wholly owned subsidiary or partially owned entity, included in the consolidation of the parent entity, to the noncontrolling interest holders. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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- Definition Decrease in noncontrolling interest (for example, but not limited to, redeeming or purchasing the interests of noncontrolling shareholders, issuance of shares (interests) by the non-wholly owned subsidiary to the parent entity for other than cash, and a buyback of shares (interest) by the non-wholly owned subsidiary from the noncontrolling interests). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 23 -URI http://asc.fasb.org/extlink&oid=68079674&loc=SL4569655-111683 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 24 -URI http://asc.fasb.org/extlink&oid=68079674&loc=SL4616395-111683 + Details
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