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Related Party Transactions (Narrative) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Equity ownership percentage of director of parent of general independent retail insurance agency | 10.00% |  |
Equity Ownership Percentage Of Son Of Director Of Parent | 10.00% |  |
Direct premiums written | $ 2,890,633 | $ 2,733,459 | $ 2,577,259
Due to Related Parties, Current | 400 | 600 |
Contributions to the Selective Insurance Group Foundation | 500 | 0 | 0
Costs and Expenses, Related Party | 2,000 | 2,000 | 400
Due to Related Parties | 1,000 | 500 |
Investments | 5,960,651 | 5,685,179 |
Related Party Insurance Agency [Member] |  |  |
Direct premiums written | 10,100 | 11,100 | 10,400
Commissions paid to Rue Insurance | $ 2,100 | 2,300 | 2,100
Vanguard Group [Member] |  |  |
Percentage Common Stock Owned by Related Party | 10.10% |  |
Related Party Transaction, Purchases from Related Party | $ 11,500 |  |
Investments | 10,500 | 0 | 0
Investment Income, Net | $ (400) |  |
BlackRock Inc [Member] |  |  |
Percentage Common Stock Owned by Related Party | 14.20% |  |
Investments | $ 800 | 500 | 500
Sale of Related Party Investments |  |  | 77,600
Related Party Transaction, Other Revenues from Transactions with Related Party |  |  | 1,100
Investment Income, Net |  |  | (2,300)
Retirement Income Plan [Member] | Vanguard Group [Member] |  |  |
Investments | 0 | 86,300 |
Investment Income, Net | (5,500) | (9,100) | (5,400)
Retirement Income Plan [Member] | BlackRock Inc [Member] |  |  |
Investments | 131,900 | 134,300 |
Investment Income, Net | $ 9,300 | $ (25,200) | $ (8,500)

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- Definition The minimum percentage of the equity of an independent insurance agency owned by a son of a director of the Parent. + References No definition available. + Details
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- Definition Costs of sales and operating expenses for the period incurred from transactions with related parties. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2) -URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227 + Details
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- Definition Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + 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://fasb.org/us-gaap/role/ref/legacyRef -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://fasb.org/us-gaap/role/ref/legacyRef -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://fasb.org/us-gaap/role/ref/legacyRef -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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