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Reinsurance Reinsurance (List of direct, assumed and ceded Reinsurance Amounts) (Details) (USD $) | 3 Months Ended | 12 Months Ended
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Dec. 31, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Premiums Written: |  |  |  |  |  |  |  |  |  |  |
Direct |  |  |  |  |  |  |  |  | $ 1,955,667,000 | $ 1,725,393,000 | $ 1,634,415,000
Assumed |  |  |  |  |  |  |  |  | 50,938,000 | 51,515,000 | 25,254,000
Ceded premiums written |  |  |  |  |  |  |  |  | (339,722,000) | (291,559,000) | (269,128,000)
Net |  |  |  |  |  |  |  |  | 1,666,883,000 | 1,485,349,000 | 1,390,541,000
Premiums Earned: |  |  |  |  |  |  |  |  |  |  |
Direct |  |  |  |  |  |  |  |  | 1,873,007,000 | 1,693,021,000 | 1,654,301,000
Assumed |  |  |  |  |  |  |  |  | 65,884,000 | 29,011,000 | 26,619,000
Ceded premiums earned |  |  |  |  |  |  |  |  | (354,772,000) | (282,719,000) | (264,322,000)
Total Net Premiums Earned | 406,853,000 | 406,225,000 | 392,212,000 | 378,829,000 | 373,427,000 | 358,963,000 | 355,580,000 | 351,343,000 | 1,584,119,000 | 1,439,313,000 | 1,416,598,000
Losses and loss expenes incurred: |  |  |  |  |  |  |  |  |  |  |
Direct |  |  |  |  |  |  |  |  | 2,394,640,000 | 1,499,340,000 | 1,102,326,000
Assumed |  |  |  |  |  |  |  |  | 29,175,000 | 20,788,000 | 14,994,000
Ceded losses and loss expenses incurred |  |  |  |  |  |  |  |  | (1,302,825,000) | (445,141,000) | (135,202,000)
Net |  |  |  |  |  |  |  |  | 1,120,990,000 | 1,074,987,000 | 982,118,000
Hurricane Sandy [Member] |  |  |  |  |  |  |  |  |  |  |
Losses and loss expenes incurred: |  |  |  |  |  |  |  |  |  |  |
Losses on a gross basis |  |  |  |  |  |  |  |  | 136,000,000 |  |
Losses on a net basis |  |  |  |  |  |  |  |  | 46,600,000 |  |
Flood loss handling fee |  |  |  |  |  |  |  |  | 18,300,000 |  |
Hurricane Sandy [Member] | National Flood Insurance Program [Member] |  |  |  |  |  |  |  |  |  |  |
Losses and loss expenes incurred: |  |  |  |  |  |  |  |  |  |  |
Ceded losses and loss expenses incurred |  |  |  |  |  |  |  |  | (1,000,000,000) |  |
Hurricane Sandy [Member] | Catatrophe excess of loss treaty [Member] |  |  |  |  |  |  |  |  |  |  |
Losses and loss expenes incurred: |  |  |  |  |  |  |  |  |  |  |
Ceded losses and loss expenses incurred |  |  |  |  |  |  |  |  | (89,400,000) |  |
Hurricane Irene [Member] |  |  |  |  |  |  |  |  |  |  |
Losses and loss expenes incurred: |  |  |  |  |  |  |  |  |  |  |
Losses on a gross basis |  |  |  |  |  |  |  |  |  | 46,500,000 |
Losses on a net basis |  |  |  |  |  |  |  |  |  | 39,600,000 |
Flood loss handling fee |  |  |  |  |  |  |  |  |  | 7,100,000 |
Hurricane Irene [Member] | National Flood Insurance Program [Member] |  |  |  |  |  |  |  |  |  |  |
Losses and loss expenes incurred: |  |  |  |  |  |  |  |  |  |  |
Ceded losses and loss expenses incurred |  |  |  |  |  |  |  |  |  | (177,000,000) |
Hurricane Irene [Member] | Catatrophe excess of loss treaty [Member] |  |  |  |  |  |  |  |  |  |  |
Losses and loss expenes incurred: |  |  |  |  |  |  |  |  |  |  |
Ceded losses and loss expenses incurred |  |  |  |  |  |  |  |  |  | $ (6,900,000) |

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- Definition Premiums assumed and earned for all property and casualty insurance assumed from other insurers as a result of reinsurance arrangements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(2)-(3) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 27 -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 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17) -URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e574992-122915 + Details
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- Definition Premiums assumed and earned by another insurer as a result of reinsurance arrangements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 16 -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 Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486662&loc=d3e27175-158546 + Details
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- Definition Premiums for property and casualty coverage ceded to another insurer under reinsurance arrangements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 27 -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 Premiums earned directly for all property and casualty insurance before adding contracts assumed from other insurers or subtracting any amounts assumed by other insurers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 27 -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 Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(3) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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- Definition Premiums written directly by insurer for all property and casualty insurance before adding contracts assumed from other insurers or subtracting any amounts assumed by other insurers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 27 -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 Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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- Definition The amount of the provision for the period for claims and benefits incurred on contract risks assumed from other (ceding) insurers under reinsurance arrangements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 27, 113 -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 The reduction in the amount of the provision for the period for claims and benefits incurred on contract risks ceded to other insurers under reinsurance arrangements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 27, 113 -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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