Related Party Transactions (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
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)

The minimum percentage of the equity of an independent insurance agency owned by a director of the Parent.

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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.

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Percentage of common stock owned by a related party

+ References

No definition available.

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- Definition

The amount of contributions made to a related party private foundation.

+ References

No definition available.

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Costs of sales and operating expenses for the period incurred from transactions with related parties.

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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
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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).

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Carrying amount as of the balance sheet date of obligations due all related parties.

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Amount after accretion (amortization) of discount (premium), and investment expense, of interest income and dividend income on nonoperating securities.

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Sum of the carrying amounts as of the balance sheet date of all investments.

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The amount of cash paid for commissions during the current period.

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Amount of transactions with related party during the financial reporting period.

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Reflects the sum of all other revenue and income realized from sales and other transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party during the period.

+ References

No definition available.

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Purchases during the period (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

+ References

No definition available.

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