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Reserve of Losses and Loss Expenses (Roll forward of Reserves for Losses and Loss Expenses) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Insurance Loss Reserves [Abstract] |  |  |  |  |  |
Gross reserves for losses and loss expenses, at beginning of year | $ 2,830,058,000 |  | $ 2,745,799,000 |  | $ 2,640,973,000 |
Less: reinsurance recoverable on unpaid loss and loss expenses, at beginning of year | 313,739,000 | [1] | 271,610,000 | [1] | 224,192,000 |
Net reserves for losses and loss expenses, at beginning of year | 2,516,319,000 |  | 2,474,189,000 |  | 2,416,781,000 |
Incurred losses and loss expense for claims occurring in the current year | 1,113,733,000 |  | 1,025,707,000 |  | 1,001,333,000 |
Incurred losses and loss expenses for claims occurring in the prior years | (38,746,000) |  | (43,589,000) |  | (29,428,000) |
Total incurred lossesand loss expenses | 1,074,987,000 |  | 982,118,000 |  | 971,905,000 |
Paid losses and loss expenses for claims occurring in current year | 440,786,000 |  | 378,650,000 |  | 330,006,000 |
Paid losses and loss expenses for claims occurring in prior years | 569,944,000 |  | 561,338,000 |  | 584,491,000 |
Total paid losses and loss expenses | 1,010,730,000 |  | 939,988,000 |  | 914,497,000 |
Acquisition of MUSIC loss and loss expenses reserves | 14,858,000 |  | 0 |  | 0 |
Net reserves for losses and loss expenses, at end of year | 2,595,434,000 |  | 2,516,319,000 |  | 2,474,189,000 |
Add: reinsurance recoverable on unpaid loss and loss expenses, at end of year | 549,490,000 | [1] | 313,739,000 | [1] | 271,610,000 | [1]
Gross reserves for losses and loss expenses, at end of year | 3,144,924,000 |  | 2,830,058,000 |  | 2,745,799,000 |
Increase of net loss and loss expense reserves | 79,100,000 |  | 42,100,000 |  | 57,400,000 |
Anticipated recoveries for salvage and subrogation claims | 67,600,000 |  | 55,000,000 |  | 58,800,000 |
Reinsurance recoverable related to the acquisition of MUSIC | $ 43,978,000 |  |  |  |  |
[1] | 1Includes $44.0 million related to the acquisition of MUSIC.

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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 The estimated cost of settling claims under the terms of the underlying insurance policies and assumed and ceded insurance contracts as of the balance sheet date, including an estimate for claims which have been incurred but not reported and the actual and estimated costs of settling claims. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11 -Subparagraph a -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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 01-5 -Paragraph 10 -Subparagraph a -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 Total provision in the period for claims incurred and costs incurred in the claim settlement process. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11 -Subparagraph b -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 (b) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition The estimated amount as of the balance sheet date to be received by an insurer from the sale of property (usually damaged) on which the insurer has paid a total claim to the insured and has obtained title to the property and the right of an insurer to pursue any course of recovery of damages, in its name or in the name of the policyholder, against a third party who is liable for costs relating to an insured event that have been paid by the insurer. Estimated salvage and subrogation is generally deducted from the liability for unpaid claims. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 30 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6482241&loc=d3e13783-158432 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-PLI -Chapter 4 -Paragraph 32 -IssueDate 2006-09-01 -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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 60 -Paragraph 18 -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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 30 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6482241&loc=d3e13779-158432 + Details
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