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CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY Unaudited - USD ($) $ in Millions | Total | Common shares and paid-in surplus | Retained earnings | AOCI, after tax | Total | Non-controlling interest | Total Equity | Foreign Currency Gain (Loss) Common shares and paid-in surplus | Foreign Currency Gain (Loss) Retained earnings | Foreign Currency Gain (Loss) AOCI, after tax | Foreign Currency Gain (Loss) Total | Foreign Currency Gain (Loss) Non-controlling interest | Foreign Currency Gain (Loss) Total Equity | 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 interest | Star & Shield LLC Common shares and paid-in surplus | Star & Shield LLC Retained earnings | Star & Shield LLC AOCI, after tax | Star & Shield LLC Total | Star & Shield LLC Non-controlling interest | Star & Shield LLC Total Equity | OneBeacon Non-controlling interest | OneBeacon Total Equity | OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] AOCI, after tax | OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] Total | OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] Total Equity | Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Common shares and paid-in surplus | Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Retained earnings | Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] AOCI, after tax | Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Total | Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Non-controlling interest | Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Total Equity | Sale of Sirius Group | Sale of Sirius Group Common shares and paid-in surplus | Sale of Sirius Group Retained earnings | Sale of Sirius Group AOCI, after tax | Sale of Sirius Group Total | Sale of Sirius Group Non-controlling interest | Sale of Sirius Group Total Equity | Sale of Tranzact Common shares and paid-in surplus | Sale of Tranzact Retained earnings | Sale of Tranzact AOCI, after tax | Sale of Tranzact Total | Sale of Tranzact Non-controlling interest | Sale of Tranzact Total Equity | OneBeacon Insurance Company Common shares and paid-in surplus | OneBeacon Insurance Company Retained earnings | OneBeacon Insurance Company AOCI, after tax | OneBeacon Insurance Company Total | OneBeacon Insurance Company Non-controlling interest | OneBeacon Insurance Company Total Equity
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Ending Balances 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 | $ 465.3 | 0.0 | 440.7 | 0.0 | 440.7 | 24.6 | 465.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in foreign currency translation and benefit plan assets and obligations | (0.4) |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 32.1 | $ 32.1 | $ 0.1 | $ 32.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 113.3 | $ 113.3 | $ 0.0 | $ 113.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total comprehensive income | 586.1 | 0.0 | 440.7 | 145.4 | 586.1 | 24.7 | 610.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  | 0.0 | (5.4) | 0.0 | (5.4) | 0.0 | (5.4) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends to non-controlling interests |  | 0.0 | 0.0 | 0.0 | 0.0 | (17.4) | (17.4) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares |  | (188.1) | (678.6) | 0.0 | (866.7) | 0.0 | (866.7) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common shares |  | 9.1 | 0.0 | 0.0 | 9.1 | 0.0 | 9.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deconsolidation of non-controlling interests |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ (250.0) | $ (250.0) | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ (78.4) | $ (78.4) |  |  |  |  |  |
Acquisition of subsidiary |  | 0.0 | 0.0 | 0.0 | 0.0 | 3.4 | 3.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Non-controlling interests, decrease from purchase of interests |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (2.7) | $ 0.0 | $ 0.0 | $ (2.7) | $ (8.8) | $ (11.5)
Issuance of shares to non-controlling interests |  | 0.0 | 0.0 | 0.0 | 0.0 | 0.3 | 0.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net contributions from non-controlling interests |  | 0.0 | 0.0 | 0.0 | 0.0 | 20.1 | 20.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization of restricted share awards |  | 13.3 | 0.0 | 0.0 | 13.3 | 0.5 | 13.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending Balances at Sep. 30, 2016 |  | 809.8 | 2,831.7 | (4.5) | 3,637.0 | 148.7 | 3,785.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning Balances at Dec. 31, 2015 |  | 978.2 | 3,075.0 | (149.9) | 3,903.3 | 454.3 | 4,357.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 113.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending Balances at Dec. 31, 2016 | 3,716.0 | 810.7 | 2,776.6 | (4.6) | 3,582.7 | 133.3 | 3,716.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning Balances at Dec. 31, 2016 | 3,716.0 | 810.7 | 2,776.6 | (4.6) | 3,582.7 | 133.3 | 3,716.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income | 581.1 | 0.0 | 604.7 | 0.0 | 604.7 | (23.6) | 581.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in foreign currency translation and benefit plan assets and obligations | 0.2 |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.4 | $ 0.4 | $ 0.1 | $ 0.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 |  |  |  |  |  |  |  |  | $ 2.9 | $ 2.9 | $ 2.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total comprehensive income | 608.1 | 0.0 | 604.7 | 3.3 | 608.0 | (23.5) | 584.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  | 0.0 | (4.6) | 0.0 | (4.6) | 0.0 | (4.6) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends to non-controlling interests |  | 0.0 | 0.0 | 0.0 | 0.0 | (17.7) | (17.7) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares |  | (147.9) | (575.9) | 0.0 | (723.8) | (5.2) | (729.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common shares |  | 1.7 | 0.0 | 0.0 | 1.7 | 0.0 | 1.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deconsolidation of non-controlling interests |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ (4.4) | $ (4.4) | $ (238.3) | $ (238.3) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Non-controlling interests, decrease from purchase of interests |  | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of shares to non-controlling interests |  | (4.1) | 0.0 | 0.0 | (4.1) | 4.7 | 0.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net contributions from non-controlling interests |  | (1.7) | 0.0 | 0.0 | (1.7) | 18.7 | 17.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization of restricted share awards |  | 10.6 | 0.0 | 0.0 | 10.6 | 0.8 | 11.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending Balances at Sep. 30, 2017 | 3,337.2 | 669.3 | 2,800.8 | (1.3) | 3,468.8 | (131.6) | 3,337.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income | 551.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in foreign currency translation and benefit plan assets and obligations | 0.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total comprehensive income | 565.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending Balances at Sep. 30, 2017 | $ 3,337.2 | $ 669.3 | $ 2,800.8 | $ (1.3) | $ 3,468.8 | $ (131.6) | $ 3,337.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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