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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 | 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 AOCI, after tax | Pension liability and other accumulated comprehensive items Total | Pension liability and other accumulated comprehensive items Non-controlling interest | Pension liability and other accumulated comprehensive items Total Equity | Prospector Turtle Fund Common shares and paid-in surplus | Prospector Turtle Fund Retained earnings | Prospector Turtle Fund AOCI, after tax | Prospector Turtle Fund Total | Prospector Turtle Fund Non-controlling interest | Prospector Turtle Fund Total Equity | Sale of Sirius Group [Member] Common shares and paid-in surplus | Sale of Sirius Group [Member] Retained earnings | Sale of Sirius Group [Member] AOCI, after tax | Sale of Sirius Group [Member] Total | Sale of Sirius Group [Member] Non-controlling interest | Sale of Sirius Group [Member] Total Equity | Sale of Sirius Group [Member] Foreign Currency Gain (Loss) Common shares and paid-in surplus | Sale of Sirius Group [Member] Foreign Currency Gain (Loss) Retained earnings | Sale of Sirius Group [Member] Foreign Currency Gain (Loss) AOCI, after tax | Sale of Sirius Group [Member] Foreign Currency Gain (Loss) Total | Sale of Sirius Group [Member] Foreign Currency Gain (Loss) Non-controlling interest | Sale of Sirius Group [Member] Foreign Currency Gain (Loss) Total Equity | Sale of Tranzact [Member] Common shares and paid-in surplus | Sale of Tranzact [Member] Retained earnings | Sale of Tranzact [Member] AOCI, after tax | Sale of Tranzact [Member] Total | Sale of Tranzact [Member] Non-controlling interest | Sale of Tranzact [Member] Total Equity | Buzzmove [Member] Common shares and paid-in surplus | Buzzmove [Member] Retained earnings | Buzzmove [Member] AOCI, after tax | Buzzmove [Member] Total | Buzzmove [Member] Non-controlling interest | Buzzmove [Member] Total Equity
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Balances 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 | $ 5.5 | 0.0 | 29.7 | 0.0 | 29.7 | (24.2) | 5.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in unrealized gains (losses) from investments in unconsolidated affiliates | (29.4) | 0.0 | 0.0 | (29.4) | (29.4) | 0.0 | (29.4) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in foreign currency translation | 0.3 |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ (62.6) | $ (62.6) | $ 0.0 | $ (62.6) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in pension liability and other accumulated comprehensive items |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.7 | $ 0.7 | $ 0.1 | $ 0.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total comprehensive income (loss) | (61.5) | 0.0 | 29.7 | (91.3) | (61.6) | (24.1) | (85.7) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  | 0.0 | (6.0) | 0.0 | (6.0) | 0.0 | (6.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends to non-controlling interests | 9.1 | 0.0 | 0.0 | 0.0 | 0.0 | (35.4) | (35.4) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares |  | (46.0) | (145.9) | 0.0 | (191.9) | 0.0 | (191.9) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuances of common shares |  | 0.9 | 0.0 | 0.0 | 0.9 | 0.0 | 0.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisition from non-controlling interests |  | (2.2) | 0.0 | 0.0 | (2.2) | (2.7) | (4.9) |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ (31.5) | $ (31.5) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net contributions from non-controlling interests |  | 0.0 | 0.0 | 0.0 | 0.0 | 12.1 | 12.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization of restricted share awards |  | 10.6 | 0.0 | 0.0 | 10.6 | (0.3) | 10.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balances at Sep. 30, 2015 |  | 998.0 | 2,888.3 | (140.8) | 3,745.5 | 460.8 | 4,206.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balances at Dec. 31, 2015 | 4,368.0 | 978.2 | 3,084.9 | (149.9) | 3,913.2 | 454.8 | 4,368.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income | 470.4 | 0.0 | 445.8 | 0.0 | 445.8 | 24.6 | 470.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in unrealized gains (losses) from investments in unconsolidated affiliates | 0.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in foreign currency translation | 0.3 |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 32.0 | $ 32.0 | $ 0.0 | $ 32.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 113.3 | $ 113.3 | $ 0.0 | $ 113.3 |  |  |  |  |  |  |  |  |  |  |  |
Net change in pension liability and other accumulated comprehensive items |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.3 | $ 0.3 | $ 0.1 | $ 0.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total comprehensive income (loss) | 591.4 | 0.0 | 445.8 | 145.6 | 591.4 | 24.7 | 616.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends declared on common shares |  | 0.0 | (5.4) | 0.0 | (5.4) | 0.0 | (5.4) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends to non-controlling interests | 1.0 | 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) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuances of common shares |  | 9.1 | 0.0 | 0.0 | 9.1 | 0.0 | 9.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest, Decrease from Deconsolidation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ (250.0) | $ (250.0) |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ (78.4) | $ (78.4) |  |  |  |  |  |
Noncontrolling Interest, Increase from Business Combination |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ 3.4 | $ 3.4
Acquisition from non-controlling 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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balances at Sep. 30, 2016 | $ 3,801.4 | $ 809.8 | $ 2,846.7 | $ (4.3) | $ 3,652.2 | $ 149.2 | $ 3,801.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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