Basis of Presentation	6 Months Ended
Jun. 30, 2018
Basis of Presentation And Significant Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation As used herein, the "Company,” “we,” “us,” or “our” refers to Selective Insurance Group, Inc. (the "Parent"), and its subsidiaries, except as expressly indicated or unless the context otherwise requires. Our interim unaudited consolidated financial statements (“Financial Statements”) have been prepared by us 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. The preparation of the Financial Statements in conformity with GAAP requires us to make estimates and assumptions that affect the reported financial statement balances, as well as 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. Certain amounts in our prior years' Financial Statements and related notes have been reclassified to conform to the 2018 presentation. Specifically, we reclassified restricted cash balances related to our participation in the National Flood Insurance Program ("NFIP") from other assets in our consolidated balance sheet into a separate line item on the face of that statement. Additionally, refer to Note 2. "Adoption of Accounting Pronouncements" below for a discussion of the retroactive restatements that are included in these financial statements in relation to the adoption of new accounting pronouncements for the treatment of restricted cash and distributions from equity method investments on the consolidated statements of cash flows. 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, 2018 (“Second Quarter 2018”) and June 30, 2017 (“Second Quarter 2017”) and the six-month periods ended June 30, 2018 ("Six Months 2018") and June 30, 2017 ("Six Months 2017"). The Financial Statements do not include all of the information and disclosures required by GAAP and the SEC for audited annual financial statements. Results of operations for any interim period are not necessarily indicative of results for a full year. Consequently, 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, 2017 (“2017 Annual Report”) filed with the SEC.

Basis of Presentation

As used herein, the "Company,” “we,” “us,” or “our” refers to Selective Insurance Group, Inc. (the "Parent"), and its subsidiaries, except as expressly indicated or unless the context otherwise requires. Our interim unaudited consolidated financial statements (“Financial Statements”) have been prepared by us 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. The preparation of the Financial Statements in conformity with GAAP requires us to make estimates and assumptions that affect the reported financial statement balances, as well as 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.

Certain amounts in our prior years' Financial Statements and related notes have been reclassified to conform to the 2018 presentation. Specifically, we reclassified restricted cash balances related to our participation in the National Flood Insurance Program ("NFIP") from other assets in our consolidated balance sheet into a separate line item on the face of that statement. Additionally, refer to Note 2. "Adoption of Accounting Pronouncements" below for a discussion of the retroactive restatements that are included in these financial statements in relation to the adoption of new accounting pronouncements for the treatment of restricted cash and distributions from equity method investments on the consolidated statements of cash flows.

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, 2018 (“Second Quarter 2018”) and June 30, 2017 (“Second Quarter 2017”) and the six-month periods ended June 30, 2018 ("Six Months 2018") and June 30, 2017 ("Six Months 2017"). The Financial Statements do not include all of the information and disclosures required by GAAP and the SEC for audited annual financial statements. Results of operations for any interim period are not necessarily indicative of results for a full year. Consequently, 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, 2017 (“2017 Annual Report”) filed with the SEC.