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Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Reconciliation of Claims Development to Liability) (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Net outstanding liabilities | $ 1,345,074 | $ 1,234,797 | $ 1,148,541 |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 3,041,694 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 585,855 | 611,200 | 551,019 | $ 571,978
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense | 143,691 |  |  |
Liability for Claims and Claims Adjustment Expense | 3,771,240 | $ 3,691,719 | $ 3,517,728 | $ 3,477,870
General Liability [Member] | Standard Commercial Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 1,092,058 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 175,276 |  |  |
General Liability [Member] | E&S Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 279,023 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 21,360 |  |  |
Workers Compensation [Member] | Standard Commercial Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 916,471 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 218,024 |  |  |
Commercial Automobile [Member] | Standard Commercial Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 464,131 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 16,745 |  |  |
Business Owners' Policies [Member] | Standard Commercial Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 83,163 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 3,926 |  |  |
Commercial Property [Member] | Standard Commercial Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 50,300 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 24,387 |  |  |
Commercial Property [Member] | E&S Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 13,519 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 389 |  |  |
Other Commercial [Member] | Standard Commercial Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 10,560 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 2,287 |  |  |
Total Standard Commercial Lines [Member] | Standard Commercial Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 2,616,683 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 440,645 |  |  |
Personal Automobile [Member] | Standard Personal Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 92,557 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 53,129 |  |  |
Homeowners [Member] | Standard Personal Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 30,728 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 999 |  |  |
Other Personal [Member] | Standard Personal Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 9,184 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 69,333 |  |  |
Standard Personal Lines [Member] | Standard Personal Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 132,469 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | 123,461 |  |  |
Total E&S Lines [Member] | E&S Lines [Member] |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Net | 292,542 |  |  |
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments | $ 21,749 |  |  |

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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + 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=65670824&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 + Details
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- Definition Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer. + 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 (cc) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 Reference 2: 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=65670824&loc=d3e14764-158437 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 + Details
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