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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 | 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 | 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 Total equity | Prospector Turtle Fund Non-controlling interests | Prospector Turtle Fund Total equity | Sale of Sirius Group | Sale of Sirius Group Non-controlling interests | Sale of Sirius Group Total equity | Tranzact Non-controlling interests | Tranzact Total equity | OneBeacon Non-controlling interests | OneBeacon Total equity | OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] AOCL, after-tax | OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] Parent | OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] Total equity | Star & Shield LLC Non-controlling interests | Star & Shield LLC Total equity
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Beginning balance at Dec. 31, 2014 |  | $ 1,034.7 | $ 3,003.0 | $ (42.1) | $ 3,995.6 | $ 542.6 | $ 4,538.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income | $ 276.7 |  | 295.2 |  | 295.2 | (18.5) | 276.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in pension liability and other accumulated comprehensive items |  |  |  |  |  |  |  | $ (73.2) | $ (73.2) |  | $ (73.2) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss), net of tax | (42.8) |  |  |  |  |  |  |  |  |  |  | $ (34.6) | $ (34.6) | $ (34.6) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Comprehensive income (loss) | 187.4 |  | 295.2 | (107.8) | 187.4 | (18.5) | 168.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  |  | (6.0) |  | (6.0) |  | (6.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends/distributions to non-controlling interests |  |  |  |  |  | (51.1) | (51.1) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuances of common shares |  | 0.9 |  |  | 0.9 |  | 0.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares | (284.2) | (67.0) | (217.2) |  | (284.2) |  | (284.2) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net contributions from non-controlling interests |  |  |  |  |  | 17.5 | 17.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition of noncontrolling interest |  | (5.3) |  |  | (5.3) | (2.7) | (8.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Redemptions of the Prospector Turtle Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (31.5) | $ (31.5) |  |  |  |  |  |  |  |  |  |  |  |
Amortization of restricted share and option awards |  | 14.9 |  |  | 14.9 | (2.0) | 12.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending balance at Dec. 31, 2015 |  | 978.2 | 3,075.0 | (149.9) | 3,903.3 | 454.3 | 4,357.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income | 409.0 |  | 401.8 |  | 401.8 | 7.2 | 409.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in pension liability and other accumulated comprehensive items |  |  |  |  |  |  |  | 31.4 | 31.4 | $ 0.3 | 31.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss), net of tax | (0.7) |  |  |  |  |  |  |  |  |  |  | $ 113.9 | $ 113.9 | $ 113.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recognition of benefit plan assets and obligations from sale of OneBeacon |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 113.3 |  |  |  |  |  |  |  |  |  |  |
Comprehensive income (loss) | 547.4 |  | 401.8 | 145.3 | 547.1 | 7.5 | 554.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  |  | (5.4) |  | (5.4) |  | (5.4) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends/distributions to non-controlling interests |  |  |  |  |  | (22.7) | (22.7) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuances of common shares |  | 9.1 |  |  | 9.1 |  | 9.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares | (887.2) | (192.4) | (694.8) |  | (887.2) |  | (887.2) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net contributions from non-controlling interests |  |  |  |  |  | 27.3 | 27.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition of noncontrolling interest |  | (2.7) |  |  | (2.7) | (8.8) | (11.5) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition of subsidiary |  |  |  |  |  | 3.3 | 3.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deconsolidation of non-controlling interests associated with sale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (250.0) | $ (250.0) | $ (78.4) | $ (78.4) |  |  |  |  |  |  |
Amortization of restricted share and option awards |  | 18.5 |  |  | 18.5 | 0.8 | 19.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending balance at Dec. 31, 2016 | 3,582.7 | 810.7 | 2,776.6 | (4.6) | 3,582.7 | 133.3 | 3,716.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income | 593.1 |  | 627.2 |  | 627.2 | (34.1) | 593.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in pension liability and other accumulated comprehensive items |  |  |  |  |  |  |  | $ 0.4 | $ 0.4 | $ 0.1 | $ 0.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss), net of tax | 0.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recognition of benefit plan assets and obligations from sale of OneBeacon |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2.9 | $ 2.9 | $ 2.9 |  |
Comprehensive income (loss) | 630.7 |  | 627.2 | 3.3 | 630.5 | (34.0) | 596.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  |  | (4.6) |  | (4.6) |  | (4.6) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends/distributions to non-controlling interests |  |  |  |  |  | (19.3) | (19.3) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuances of common shares |  | 1.7 |  |  | 1.7 |  | 1.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares | (723.9) | (147.9) | (576.0) |  | (723.9) | (5.2) | (729.1) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net contributions from non-controlling interests |  | (4.6) |  |  | (4.6) | 30.2 | 25.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of shares of non-controlling interests |  | (4.1) |  |  | (4.1) | 5.2 | 1.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deconsolidation of non-controlling interests associated with sale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (238.3) | $ (238.3) |  |  |  | $ (4.4) | $ (4.4)
Amortization of restricted share and option awards |  | 14.8 |  |  | 14.8 | 0.8 | 15.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending balance at Dec. 31, 2017 | $ 3,492.5 | $ 670.6 | $ 2,823.2 | $ (1.3) | $ 3,492.5 | $ (131.7) | $ 3,360.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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