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CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - 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 | 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 | 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 | 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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Beginning Balances at Dec. 31, 2015 |  | $ 978.2 | $ 3,084.9 | $ (149.9) | $ 3,913.2 | $ 454.8 | $ 4,368.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income | $ 381.8 | 0.0 | 354.1 | 0.0 | 354.1 | 27.7 | 381.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in foreign currency translation and benefit plan assets and obligations | (0.1) |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 32.1 | $ 32.1 | $ 0.0 | $ 32.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 113.3 | $ 113.3 | $ 0.0 | $ 113.3 |  |  |  |  |  |  |  |  |  |  |  |  |
Total comprehensive income | 499.5 | 0.0 | 354.1 | 145.4 | 499.5 | 27.7 | 527.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 | (12.1) | (12.1) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares |  | (120.4) | (427.0) | 0.0 | (547.4) | 0.0 | (547.4) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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) |  |  |  |  |  |
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 | 11.9 | 11.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization of restricted share awards |  | 8.9 | 0.0 | 0.0 | 8.9 | 0.4 | 9.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending Balances at Jun. 30, 2016 |  | 873.1 | 3,006.6 | (4.5) | 3,875.2 | 224.2 | 4,099.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning Balances at Dec. 31, 2015 |  | 978.2 | 3,084.9 | (149.9) | 3,913.2 | 454.8 | 4,368.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of Sirius Group |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 113.3 |  |  |  |  |  |  |  |  |  |  |  |
Ending Balances at Dec. 31, 2016 | 3,737.2 | 810.7 | 2,797.2 | (4.6) | 3,603.3 | 133.9 | 3,737.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income | 36.4 | 0.0 | 49.8 | 0.0 | 49.8 | (13.4) | 36.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in foreign currency translation and benefit plan assets and obligations | 1.4 |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 1.6 | $ 1.6 | $ 0.1 | $ 1.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total comprehensive income | 51.5 | 0.0 | 49.8 | 1.6 | 51.4 | (13.3) | 38.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 | (12.1) | (12.1) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares |  | (2.0) | (7.2) | 0.0 | (9.2) | (5.2) | (14.4) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common shares |  | 1.6 | 0.0 | 0.0 | 1.6 | 0.0 | 1.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deconsolidation of non-controlling interests |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ (4.4) | $ (4.4) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance of shares to non-controlling interests |  | (4.8) | 0.0 | 0.0 | (4.8) | 4.8 | 0.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net contributions from non-controlling interests |  | 0.0 | 0.0 | 0.0 | 0.0 | 12.6 | 12.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization of restricted share awards |  | 9.6 | 0.0 | 0.0 | 9.6 | 0.5 | 10.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending Balances at Jun. 30, 2017 | $ 3,764.1 | $ 815.1 | $ 2,835.2 | $ (3.0) | $ 3,647.3 | $ 116.8 | $ 3,764.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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