Related Party Transactions (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investments	$ 6,688,654	$ 5,960,651
Direct premiums written	3,084,451	2,890,633	$ 2,733,459
Contributions to the Selective Insurance Group Foundation	1,300	500	0
Costs and Expenses, Related Party	2,200	2,000	2,000
Due to Related Parties	$ 1,100	1,000
Related Party Insurance Agency [Member]
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	$ 11,000	10,100	11,100
Commissions paid to Rue Insurance	2,000	2,100	2,300
Due to Related Parties, Current	$ 300	$ 400
Vanguard Group [Member]
Percentage Common Stock Owned by Related Party	9.40%	10.10%
Related Party Transaction, Purchases from Related Party		$ 11,500
Sale of Related Party Investments	$ 11,800
Investment Income, Dividend	200	400
Related Party Capital Gains (Losses)	$ 1,300
BlackRock Inc [Member]
Percentage Common Stock Owned by Related Party	11.70%
Investments			500
Related Party Transaction, Purchases from Related Party	$ 21,700	41,400
Sale of Related Party Investments	59,500
Net realized and unrealized gain (loss)	5,700	(3,600)
Investment Income (Loss), Net	800
Retirement Income Plan [Member] \| Vanguard Group [Member]
Investments			86,300
Related Party Transaction, Purchases from Related Party		8,400	5,200
Sale of Related Party Investments		85,400
Investment Income (Loss), Net		(5,500)	9,100
Retirement Income Plan [Member] \| BlackRock Inc [Member]
Investments	144,200	131,900
Related Party Transaction, Purchases from Related Party	19,700	132,500	10,000
Sale of Related Party Investments	44,100	125,600	4,100
Investment Income (Loss), Net	$ 36,700	$ 9,300	$ 25,200

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

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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

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The net capital gain (loss) during the period with transactions from a related party.

+ References

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

+ References

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

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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
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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 of dividend income on nonoperating securities.

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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.

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

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