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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 | 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) AOCI, 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 AOCI, 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] | Prospector Turtle Fund Common shares and paid-in surplus | Prospector Turtle Fund Retained earnings | Prospector Turtle Fund AOCI, after-tax | Prospector Turtle Fund Parent | Prospector Turtle Fund Non-controlling Interests | Prospector Turtle Fund Total Equity includes Noncontrolling Interest [Member]
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Beginning balance at Dec. 31, 2012 |  | $ 1,057.2 | $ 2,542.5 | $ 131.9 | $ 3,731.6 | $ 526.3 | $ 4,257.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (Decrease) in Shareholders' Equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income | $ 309.1 | 0.0 | 321.6 | 0.0 | 321.6 | (12.5) | 309.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in unrealized (losses) gains on investments in unconsolidated affiliates, after tax | (98.1) | 0.0 | 0.0 | (98.1) | (98.1) | 0.0 | (98.1) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income (loss), after-tax |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 15.6 | $ 15.6 | $ 5.2 | $ 20.8 |  |  |  |  |  |
Change in foreign currency translation, net of tax | 2.7 |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 2.7 | $ 2.7 | $ 0.0 | $ 2.7 |  |  |  |  |  |  |  |  |  |  |  |
Net change in pension liability and other accumulated comprehensive items | 20.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Comprehensive income | 247.0 | 0.0 | 321.6 | (79.8) | 241.8 | (7.3) | 234.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 | (39.7) | (39.7) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuances of common shares |  | 1.0 | 0.0 | 0.0 | 1.0 | 0.0 | 1.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchases and retirements of common shares | (79.8) | (23.8) | (56.0) | 0.0 | (79.8) | 0.0 | (79.8) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest, Increase from Subsidiary Equity Issuance |  | 0.0 | 0.0 | 0.0 | 0.0 | 11.5 | 11.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization of restricted share and option awards |  | 16.7 | 0.0 | 0.0 | 16.7 | 0.9 | 17.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending 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 (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) |  |  |  |  |  |
Change in foreign currency translation, net of tax | (168.5) |  |  |  |  |  |  | 0.0 | 0.0 | (168.2) | (168.2) | (0.3) | (168.5) |  |  |  |  |  |  |  |  |  |  |  |
Net change in pension liability and other accumulated comprehensive items | (11.7) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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, Decrease from Redemptions or Purchase of Interests |  | 0.0 | 0.0 | 0.0 | 0.0 | (122.1) | (122.1) |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ 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 | 3,995.7 | 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 (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 |  |  |  |  |  |
Change in foreign currency translation, net of tax | (65.8) |  |  |  |  |  |  | $ 0.0 | $ 0.0 | $ (65.8) | $ (65.8) | $ 0.0 | $ (65.8) |  |  |  |  |  |  |  |  |  |  |  |
Net change in pension liability and other accumulated comprehensive items | 0.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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