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Related Party Transactions (Narrative) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Equity ownership percentage of director of parent of general independent retail insurance agency | 10.00% |  |
Direct premiums written | $ 2,733,459 | $ 2,577,259 | $ 2,403,519
Due to Related Parties, Current | 600 | 700 |
Contributions to the Selective Insurance Group Foundation | $ 100 | 0 | 1,000
Percentage Common Stock Owned by Related Party | 12.80% |  |
Costs and Expenses, Related Party | $ 2,000 | 400 |
Due to Related Parties | 500 | 400 |
Related Party Insurance Agency [Member] |  |  |
Direct premiums written | 11,100 | 10,400 | 9,600
Commissions paid to Rue Insurance | $ 2,300 | $ 2,100 | $ 1,700

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- Definition Costs of sales and operating expenses for the period incurred from transactions with related parties. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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- Definition Carrying amount as of the balance sheet date of obligations due all related parties. For classified balance sheets, represents the current portion of such liabilities (due within one year or within the normal operating cycle if longer). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(k)(1)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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