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Statutory Financial Information, Capital Requirements, and Restrictions on Dividends and Transfers of Funds - Schedule of Insurance Subsidiaries Statutory Surplus Data (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Statutory Accounting Practices [Line Items]
Unassigned Surplus	$ 2,337.2	$ 2,176.7
Statutory Surplus	2,902.8	2,742.3
Statutory Net Income	178.3	318.8	$ 264.4
Selective Insurace Company of America (SICA)
Statutory Accounting Practices [Line Items]
Unassigned Surplus	832.3	773.6
Statutory Surplus	997.5	938.8
Statutory Net Income	76.8	120.4	103.1
Selective Way Insurance Company ("SWIC")
Statutory Accounting Practices [Line Items]
Unassigned Surplus	537.1	513.5
Statutory Surplus	593.1	569.3
Statutory Net Income	36.3	64.1	69.6
Selective Insurance Company of South Carolina (SICSC)
Statutory Accounting Practices [Line Items]
Unassigned Surplus	219.9	201.3
Statutory Surplus	254.1	235.6
Statutory Net Income	13.3	26.0	17.0
Selective Insurance Company of the Southeast (SICSE)
Statutory Accounting Practices [Line Items]
Unassigned Surplus	171.9	155.1
Statutory Surplus	199.5	182.7
Statutory Net Income	11.2	21.6	14.2
Selective Insurance Company of New York (SICNY)
Statutory Accounting Practices [Line Items]
Unassigned Surplus	162.0	152.4
Statutory Surplus	189.7	180.1
Statutory Net Income	7.1	16.4	13.2
Selective Insurance Company of New England ("SICNE")
Statutory Accounting Practices [Line Items]
Unassigned Surplus	49.2	45.5
Statutory Surplus	80.3	76.7
Statutory Net Income	3.3	8.3	5.1
Selective Auto Insurance Company of New Jersey ("SAICNJ")
Statutory Accounting Practices [Line Items]
Unassigned Surplus	127.5	119.3
Statutory Surplus	172.4	164.2
Statutory Net Income	7.4	16.2	12.7
Mesa Underwriters Specialty Insurance Company ("MUSIC")
Statutory Accounting Practices [Line Items]
Unassigned Surplus	72.3	64.3
Statutory Surplus	141.8	133.7
Statutory Net Income	8.7	14.4	9.7
Selective Casualty Insurance Company ("SCIC")
Statutory Accounting Practices [Line Items]
Unassigned Surplus	117.2	107.8
Statutory Surplus	193.6	184.3
Statutory Net Income	10.7	24.0	14.0
Selective Fire and Casualty Insurance Company ("SFCIC")
Statutory Accounting Practices [Line Items]
Unassigned Surplus	47.8	43.9
Statutory Surplus	80.8	76.9
Statutory Net Income	$ 3.5	$ 7.4	$ 5.8

X

- Definition

The amount of statutory unassigned surplus as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country.

+ References

No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

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- Definition

The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

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-Name Accounting Standards Codification
-Topic 944
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
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- Definition

Amount of net income for the period determined using accounting principles prescribed or permitted by insurance regulators.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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