Indebtedness (Short Term Debt Activity) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Short-term Debt	$ 302,000	$ 0
Short-term Debt [Member]
Short-term Debt [Line Items]
Short-term Debt	302,000	0
Zero Point Sixty Eight Percent Borrowing from FHLBNY Due September 18, 2020. [Member]
Short-term Debt [Line Items]
Short-term Debt	85,000	0
Zero Point Seventy Eight Percent Borrowing from FHLBNY Due September 14, 2020. [Member]
Short-term Debt [Line Items]
Short-term Debt	100,000	0
Zero Point Fifty Eight Percent Borrowing from FHLBI Due December 14, 2020. [Member]
Short-term Debt [Line Items]
Short-term Debt	67,000	$ 0
Selective Insurance Group [Member]
Short-term Debt [Line Items]
Line of Credit	$ 50,000
Line of Credit Interest Rate	2.244%
Selective Insurance Company of South Carolina [Member] \| Zero Point Fifty Eight Percent Borrowing from FHLBI Due December 14, 2020. [Member]
Short-term Debt [Line Items]
Federal Home Loan Bank Advances	$ 28,000
Federal Home Loan Bank, Advances, Interest Rate	0.58%
Selective Insurance Company of the Southeast [Member] \| Zero Point Fifty Eight Percent Borrowing from FHLBI Due December 14, 2020. [Member]
Short-term Debt [Line Items]
Federal Home Loan Bank Advances	$ 39,000
Federal Home Loan Bank, Advances, Interest Rate	0.58%
Selective Insurance Company of America [Member] \| Zero Point Sixty Eight Percent Borrowing from FHLBNY Due September 18, 2020. [Member]
Short-term Debt [Line Items]
Federal Home Loan Bank Advances	$ 85,000
Federal Home Loan Bank, Advances, Interest Rate	0.68%
Selective Insurance Company of America [Member] \| Zero Point Seventy Eight Percent Borrowing from FHLBNY Due September 14, 2020. [Member]
Short-term Debt [Line Items]
Federal Home Loan Bank Advances	$ 100,000
Federal Home Loan Bank, Advances, Interest Rate	0.78%
Selective Insurance Company of America [Member] \| One Point Eighty One Percent Borrowing from FHLBNY Due March 18, 2020 [Member]
Short-term Debt [Line Items]
Federal Home Loan Bank, Advances, Interest Rate	1.81%

Amount of Federal Home Loan Bank (FHLBank) advances to member financial institutions.

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Interest rate of advances made and reported by Federal Home Loan Bank (FHLBank).

+ References

No definition available.

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The effective interest rate at the end of the reporting period.

+ References

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- Definition

The carrying value as of the balance sheet date of the current portion of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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