Basis of Presentation	6 Months Ended
Jun. 30, 2021
Basis of Presentation And Significant Accounting Policies [Abstract]
Basis of Presentation	Basis of PresentationThe words "Company,” “we,” “us,” or “our” refer to Selective Insurance Group, Inc. (the "Parent") and its subsidiaries, except as expressly indicated or the context requires otherwise. We have prepared our interim unaudited consolidated financial statements (“Financial Statements”) in conformity with U.S. generally accepted accounting principles (“GAAP”) and the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) regarding interim financial reporting. These require us to make estimates and assumptions that affect the reported financial statement balances and the disclosure of contingent assets and liabilities. Actual results could differ from those estimates. All significant intercompany accounts and transactions between the Parent and its subsidiaries are eliminated in consolidation.Our Financial Statements reflect all adjustments that, in our opinion, are normal, recurring, and necessary for a fair presentation of our results of operations and financial condition. Our Financial Statements cover the second quarters ended June 30, 2021 (“Second Quarter 2021”) and June 30, 2020 (“Second Quarter 2020”) and the six-month periods ended June 30, 2021 (“Six Months 2021”) and June 30, 2020 (“Six Months 2020”). Our Financial Statements do not include all information and disclosures required by GAAP and the SEC for audited annual financial statements. Because results of operations for any interim period are not necessarily indicative of results for a full year, our Financial Statements should be read in conjunction with the consolidated financial statements contained in our Annual Report on Form 10-K for the year ended December 31, 2020 (“2020 Annual Report”) filed with the SEC.

Basis of Presentation

6 Months Ended

Jun. 30, 2021

Basis of Presentation And Significant Accounting Policies [Abstract]

Basis of PresentationThe words "Company,” “we,” “us,” or “our” refer to Selective Insurance Group, Inc. (the "Parent") and its subsidiaries, except as expressly indicated or the context requires otherwise. We have prepared our interim unaudited consolidated financial statements (“Financial Statements”) in conformity with U.S. generally accepted accounting principles (“GAAP”) and the rules and regulations of the U.S. Securities and Exchange Commission (“SEC”) regarding interim financial reporting. These require us to make estimates and assumptions that affect the reported financial statement balances and the disclosure of contingent assets and liabilities. Actual results could differ from those estimates. All significant intercompany accounts and transactions between the Parent and its subsidiaries are eliminated in consolidation.Our Financial Statements reflect all adjustments that, in our opinion, are normal, recurring, and necessary for a fair presentation of our results of operations and financial condition. Our Financial Statements cover the second quarters ended June 30, 2021 (“Second Quarter 2021”) and June 30, 2020 (“Second Quarter 2020”) and the six-month periods ended June 30, 2021 (“Six Months 2021”) and June 30, 2020 (“Six Months 2020”). Our Financial Statements do not include all information and disclosures required by GAAP and the SEC for audited annual financial statements. Because results of operations for any interim period are not necessarily indicative of results for a full year, our Financial Statements should be read in conjunction with the consolidated financial statements contained in our Annual Report on Form 10-K for the year ended December 31, 2020 (“2020 Annual Report”) filed with the SEC.