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Reserve of Losses and Loss Expenses (Rollforward of Reserves for Losses and Loss Expenses) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Gross reserves for losses and loss expenses, at beginning of year | $ 3,144,924,000 |  | $ 2,830,058,000 |  | $ 2,745,799,000
Less: reinsurance recoverable on unpaid loss and loss expenses, at beginning of year | 549,490,000 | [1] | 313,739,000 |  | 271,610,000
Net reserves for losses and loss expenses, at beginning of year | 2,595,434,000 |  | 2,516,319,000 |  | 2,474,189,000
Incurred losses and loss expense for claims occurring in the current year | 1,146,591,000 |  | 1,113,733,000 |  | 1,025,707,000
Incurred losses and loss expenses for claims occurring in the prior years | (25,601,000) |  | (38,746,000) |  | (43,589,000)
Total incurred lossesand loss expenses | 1,120,990,000 |  | 1,074,987,000 |  | 982,118,000
Paid losses and loss expenses for claims occurring in current year | 424,496,000 |  | 440,786,000 |  | 378,650,000
Paid losses and loss expenses for claims occurring in prior years | 632,742,000 |  | 569,944,000 |  | 561,338,000
Total paid losses and loss expenses | 1,057,238,000 |  | 1,010,730,000 |  | 939,988,000
Acquisition of MUSIC loss and loss expense reserves | 0 |  | 14,858,000 |  | 0
Net reserves for losses and loss expenses, at end of year | 2,659,186,000 |  | 2,595,434,000 |  | 2,516,319,000
Add: reinsurance recoverable on unpaid loss and loss expenses, at end of year | 1,409,755,000 |  | 549,490,000 | [1] | 313,739,000
Gross reserves for losses and loss expenses, at end of year | 4,068,941,000 |  | 3,144,924,000 |  | 2,830,058,000
Reinsurance recoverable related to the acquisition of MUSIC |  |  | 43,978,000 |  |
Increase of net loss and loss expense reserves | 63,800,000 |  | 79,100,000 |  | 42,100,000
Anticipated recoveries for salvage and subrogation claims | $ 62,200,000 |  | $ 67,600,000 |  | $ 55,000,000
[1] | 1 Includes $44.0 million related to the acquisition of MUSIC at December 31, 2011.

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- Definition The amount of reinsurance recoverables, net recognized as of the acquisition date. + References No definition available. + Details
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- Definition The amount of reserve for losses and loss expenses, net of reinsurance recoverables, recognized as of the acquisition date. + References No definition available. + Details
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- Definition The increase or decrease during the period in the liability for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims. + References No definition available. + Details
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- Definition Total amount of payments made during the reporting period to settle insured claims and pay costs incurred in the claims settlement process. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Amount of payments made in the reporting period to settle claims incurred in the current period and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11 -Subparagraph c -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition Amount of payments made in the reporting period to settle claims incurred in prior periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11 -Subparagraph c -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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