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Statutory Financial Information, Capital Requirements and Restrictions on Dividends and Transfers of Funds (Statutory Financial Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statutory unassigned surplus	$ 2,176.7	$ 1,908.1
Statutory surplus	2,742.3	2,473.7
Statutory net income	318.8	264.4	$ 338.3
Selective Insurace Company of America (SICA) [Member]
Statutory unassigned surplus	773.6	686.6
Statutory surplus	938.8	851.8
Statutory net income	120.4	103.1	134.7
Selective Way Insurance Company (SWIC) [Member]
Statutory unassigned surplus	513.5	461.9
Statutory surplus	569.3	517.8
Statutory net income	64.1	69.6	74.5
Selective Insurance Company of South Carolina (SICSC) [Member]
Statutory unassigned surplus	201.3	164.1
Statutory surplus	235.6	198.4
Statutory net income	26.0	17.0	24.2
Selective Insurance Company of the Southeast (SICSE) [Member]
Statutory unassigned surplus	155.1	135.9
Statutory surplus	182.7	163.5
Statutory net income	21.6	14.2	19.4
Selective Insurance Company of New York (SICNY) [Member]
Statutory unassigned surplus	152.4	137.4
Statutory surplus	180.1	165.1
Statutory net income	16.4	13.2	18.6
Selective Insurance Company of New England (SICNE) [Member]
Statutory unassigned surplus	45.5	38.2
Statutory surplus	76.7	69.3
Statutory net income	8.3	5.1	7.5
Selective Auto Insurance Company of New Jersey (SAICNJ) [Member]
Statutory unassigned surplus	119.3	102.2
Statutory surplus	164.2	147.1
Statutory net income	16.2	12.7	16.7
Mesa Underwriting Specialty Insurance Company (MUSIC) [Member]
Statutory unassigned surplus	64.3	52.8
Statutory surplus	133.7	122.3
Statutory net income	14.4	9.7	13.9
Selective Casualty Insurance Company (SCIC) [Member]
Statutory unassigned surplus	107.8	91.4
Statutory surplus	184.3	167.9
Statutory net income	24.0	14.0	20.6
Selective Fire and Casualty Insurance Company (SFCIC) [Member]
Statutory unassigned surplus	43.9	37.6
Statutory surplus	76.9	70.5
Statutory net income	$ 7.4	$ 5.8	$ 8.2

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.

+ Details

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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//1943274/2147479440/944-210-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 944
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479989/944-505-50-1

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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
-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//1943274/2147479440/944-210-S99-1

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