Related Party Transactions (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments	$ 7,505,599	$ 6,688,654
Direct premiums written	3,204,512	3,084,451	$ 2,890,633
Contributions to the Selective Insurance Group Foundation	500	1,300	500
Costs and Expenses, Related Party - BlackRock	2,000	2,200	2,000
Due to Related Parties	$ 1,300	1,100
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	11,000	10,100
Commissions paid to Rue Insurance	1,800	2,000	2,100
Due to Related Parties, Current	$ 200	300
BlackRock Inc [Member]
Percentage Common Stock Owned by Related Party	11.20%
Related Party Transaction, Purchases from Related Party	$ 62,200	21,700	41,400
Sale of Related Party Investments		59,500
Net realized and unrealized gain (loss)	(200)	5,700	(3,600)
Investment Income (Loss), Net	400	800
Retirement Income Plan [Member] \| BlackRock Inc [Member]
Investments	191,800	144,200
Related Party Transaction, Purchases from Related Party	56,700	19,700	132,500
Sale of Related Party Investments	44,900	44,100	125,600
Investment Income (Loss), Net	$ 35,800	$ 36,700	$ 9,300

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

+ 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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Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
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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.

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

+ References

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