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(Rollforward of Fair Value Measurements by Level) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended |  | 9 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12 Months Ended | 9 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  |  | 9 Months Ended |  |
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Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2014 | Sep. 30, 2013 | Dec. 31, 2013 | Sep. 30, 2014 Level 1 Inputs | Sep. 30, 2013 Level 1 Inputs | Sep. 30, 2014 Level 2 Inputs | Sep. 30, 2013 Level 2 Inputs | Sep. 30, 2014 Fair Value | Sep. 30, 2013 Fair Value | Sep. 30, 2014 Fixed maturity investments. Level 3 Inputs | Sep. 30, 2013 Fixed maturity investments. Level 3 Inputs | Sep. 30, 2014 Common equity securities Level 3 Inputs | Sep. 30, 2013 Common equity securities Level 3 Inputs | Sep. 30, 2014 Convertible Debt Securities Level 3 Inputs | Sep. 30, 2013 Convertible Debt Securities Level 3 Inputs | Sep. 30, 2014 Other long-term investments Level 3 Inputs | Sep. 30, 2013 Other long-term investments Level 3 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Level 1 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Level 1 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Level 2 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Level 2 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Level 3 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Level 3 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Fair Value | Dec. 31, 2013 Fair value measured on a recurring basis Fair Value | Sep. 30, 2014 Fair value measured on a recurring basis Fixed maturity investments. Level 1 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Fixed maturity investments. Level 1 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Fixed maturity investments. Level 2 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Fixed maturity investments. Level 2 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Fixed maturity investments. Level 3 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Fixed maturity investments. Level 3 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Fixed maturity investments. Fair Value | Dec. 31, 2013 Fair value measured on a recurring basis Fixed maturity investments. Fair Value | Sep. 30, 2014 Fair value measured on a recurring basis Convertible Debt Securities Level 1 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Convertible Debt Securities Level 1 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Convertible Debt Securities Level 2 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Convertible Debt Securities Level 2 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Convertible Debt Securities Level 3 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Convertible Debt Securities Level 3 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Convertible Debt Securities Fair Value | Dec. 31, 2013 Fair value measured on a recurring basis Convertible Debt Securities Fair Value | Sep. 30, 2014 Fair value measured on a recurring basis Other long-term investments Level 1 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Other long-term investments Level 1 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Other long-term investments Level 2 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Other long-term investments Level 2 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Other long-term investments Level 3 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Other long-term investments Level 3 Inputs | Sep. 30, 2013 Fair value measured on a recurring basis Other long-term investments Level 3 Inputs | Dec. 31, 2012 Fair value measured on a recurring basis Other long-term investments Level 3 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Other long-term investments Fair Value | Dec. 31, 2013 Fair value measured on a recurring basis Other long-term investments Fair Value | Dec. 31, 2013 Fair value measured on a recurring basis Forward Contracts Level 2 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Forward Contracts Level 3 Inputs | Sep. 30, 2013 Fair value measured on a recurring basis Forward Contracts Level 3 Inputs | Sep. 30, 2013 Fair value measured on a recurring basis Short-term investments Level 1 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Short-term investments Level 1 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Short-term investments Level 1 Inputs | Dec. 31, 2012 Fair value measured on a recurring basis Short-term investments Level 1 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Short-term investments Level 2 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Short-term investments Level 2 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Short-term investments Level 3 Inputs | Dec. 31, 2013 Fair value measured on a recurring basis Short-term investments Level 3 Inputs | Sep. 30, 2014 Fair value measured on a recurring basis Short-term investments Fair Value | Dec. 31, 2013 Fair value measured on a recurring basis Short-term investments Fair Value | Sep. 30, 2014 Segment, Discontinued Operations | Dec. 31, 2013 Segment, Discontinued Operations | Sep. 30, 2013 Segment, Discontinued Operations | Dec. 31, 2012 Segment, Discontinued Operations | Sep. 30, 2014 Prospector Offshore Fund Ltd [Member] Fair value measured on a recurring basis Investment related liabilities [Member] | Sep. 30, 2013 Prospector Offshore Fund Ltd [Member] Fair value measured on a recurring basis Investment related liabilities [Member] | Sep. 30, 2013 Forward Contracts Fair value measured on a recurring basis Level 3 Inputs | Dec. 31, 2012 Forward Contracts Fair value measured on a recurring basis Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balance at January 1, 2014 |  |  |  |  |  | $ 1,376.7 | $ 1,355.1 | $ 4,982.2 | $ 5,206.1 | $ 6,766.5 | $ 6,950.7 | $ 93.0 | $ 92.9 | $ 46.1 | $ 37.3 | $ 6.1 | $ 0 | $ 262.4 | $ 259.3 | $ 1,923.8 | $ 1,998.2 | $ 4,786.2 | $ 4,996.6 | $ 446.5 | $ 407.6 | $ 7,156.5 | $ 7,402.4 | $ 219.2 | $ 340.3 | $ 4,670.7 | $ 4,833.5 | $ 99.5 | $ 93.0 | $ 4,989.4 | $ 5,266.8 | $ 0 | $ 0 | $ 38.0 | $ 74.4 | $ 10.9 | $ 6.1 | $ 48.9 | $ 80.5 | $ 0 | $ 0 | $ 0 | $ 0 | $ 297.2 | $ 262.4 |  |  | $ 297.2 | $ 262.4 |  |  |  |  | $ 833.9 | $ 621.5 |  | $ 1.0 | $ 14.4 | $ 0 | $ 0 | $ 834.9 | $ 635.9 |  |  |  |  |  |  |  |
Total realized and unrealized gains |  |  |  |  |  | 57.5 | 140.6 | 115.0 | (70.7) | 198.2 | 80.6 | 3.1 | (3.3) | 3.7 | 0 | 3.3 | 0 | 15.6 | 14.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Foreign currency gains (losses) through OCI |  |  |  |  |  | (16.8) | 1.4 | (142.1) | 17.5 | (163.3) | 18.6 | (0.9) | 0.5 | 0 | 0 | 0 | 0 | (3.5) | (0.8) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amortization/Accretion |  |  |  |  |  | (0.4) | (0.9) | (33.2) | (39.7) | (33.7) | (40.6) | (0.1) | 0 | 0 | 0 | 0 | 0 | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchases |  |  |  |  |  | 1,261.6 | 735.9 | 2,595.3 | 3,014.7 | 3,985.5 | 3,875.3 | 80.0 | 32.4 | 0 | 0.5 | 1.5 | 0 | 47.1 | 91.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sales |  |  |  |  |  | (1,585.8) | (877.9) | (2,815.2) | (3,309.7) | (4,436.3) | (4,279.4) | 0 | (0.7) | (10.9) | 0 | 0 | 0 | (24.4) | (91.1) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net change in investments related to (sales) purchases of consolidated affiliates |  |  |  |  |  | (2.7) | 14.5 | 7.4 | 2.7 | 4.7 | 17.2 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Transfers in |  |  |  |  |  | 0 | 1.8 | 75.8 | 32.1 | 75.8 | 124.2 | 0 | 90.3 | 0 | 0 | 0 | 0 | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Transfers out |  |  |  |  |  | (0.2) | 0 | 0 | (91.1) | (75.8) | (124.2) | (75.6) | (32.1) | 0 | (1.0) | 0 | 0 | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balance at September 30, 2014 |  |  |  |  |  | 1,089.9 | 1,370.5 | 4,785.2 | 4,761.9 | 6,321.6 | 6,622.4 | 99.5 | 180.0 | 38.9 | 36.8 | 10.9 | 0 | 297.2 | 273.2 | 1,923.8 | 1,998.2 | 4,786.2 | 4,996.6 | 446.5 | 407.6 | 7,156.5 | 7,402.4 | 219.2 | 340.3 | 4,670.7 | 4,833.5 | 99.5 | 93.0 | 4,989.4 | 5,266.8 | 0 | 0 | 38.0 | 74.4 | 10.9 | 6.1 | 48.9 | 80.5 | 0 | 0 | 0 | 0 | 297.2 | 262.4 |  |  | 297.2 | 262.4 |  |  |  |  | 833.9 | 621.5 |  | 1.0 | 14.4 | 0 | 0 | 834.9 | 635.9 |  |  |  |  |  |  |  |
Carrying value of investment accounted for using the equity method |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 23.5 | 26.6 | 26.9 | 35.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (0.1) | (0.1)
Realized and unrealized gain | 14.2 | 27.0 | 153.1 | 63.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net realized and unrealized investment gains | 20.8 | 28.2 | 198.5 | 66.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.6 | 14.1 |  |
Short-term investments, at amortized cost (which approximates fair value) | 834.9 |  | 834.9 |  | 635.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 648.0 | 834.9 | 635.9 | 630.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Realized and unrealized gains (losses) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (0.1) | 0.2 | (0.1) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fixed maturity investments reclassified to assets held for sale |  |  |  |  | 236.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 203.9 | 236.3 | 239.7 | 338.1 |  |  |  |
Maximum price difference provided by pricing services |  |  | $ 1.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition The sum of the periodic adjustments of the differences between securities' face values and purchase prices that are charged against earnings. This is called accretion if the security was purchased at a discount and amortization if it was purchased at premium. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5618551-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959 + Details
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- Definition Fair value portion of investments accounted under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6903542&loc=d3e32014-111567 + Details
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- Definition This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c),9(a)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27405-111563 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Article 5 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (c)-(e) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph a, f, g -Article 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 + Details
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- Definition The cash outflow associated with the purchase of all investments (debt, security, other) during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585 + Details
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- Definition The cash inflow associated with the sale, maturities, repayments and calls of securities classified as trading securities and held for investment purposes. Excludes proceeds from trading securities purchased and held principally for the purpose of selling them in the near term (thus held for only a short period of time). + References No definition available. + Details
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- Definition Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph g -Article 7 + Details
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- Definition This element represents the amount of the total realized and unrealized (holding) gains or losses for the period which are included in the statement of income. + References No definition available. + Details
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- Definition This element represents transfers in to one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis. + References No definition available. + Details
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- Definition This element represents transfers out of one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis. + References No definition available. + Details
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- Definition Represents the foreign transaction gain (loss) recognized through other comprehensive income. + References No definition available. + Details
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- Definition Represents the maximum price difference provided by the pricing services between the market price and the actual sale price of securities, on the basis of which outliers are considered. + References No definition available. + Details
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- Definition Net change in investments related to purchases and sales of consolidated/unconsolidated affiliates + References No definition available. + Details
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- Definition Net Realized and Unrealized Gain (Loss) on Trading Securities, Net of Tax + References No definition available. + Details
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Period Type: | duration

X
- Definition Represents the fair value of trading debt securities re-classified to assets held for sale. + References No definition available. + Details
Name: | wtm_TradingSecuritiesDebtReclassifiedToAssetsHeldForSale
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant