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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, 2020	Dec. 31, 2019	Dec. 31, 2018
Statutory unassigned surplus	$ 1,559.1	$ 1,396.8
Statutory surplus	2,124.7	1,932.4
Statutory net income	242.4	292.7	$ 210.6
Selective Insurace Company of America (SICA) [Member]
Statutory unassigned surplus	574.2	525.9
Statutory surplus	739.4	680.1
Statutory net income	81.8	113.9	78.0
Selective Way Insurance Company (SWIC) [Member]
Statutory unassigned surplus	374.0	339.2
Statutory surplus	430.0	388.2
Statutory net income	54.0	59.2	47.5
Selective Insurance Company of South Carolina (SICSC) [Member]
Statutory unassigned surplus	148.6	132.6
Statutory surplus	182.8	163.8
Statutory net income	20.8	23.9	16.5
Selective Insurance Company of the Southeast (SICSE) [Member]
Statutory unassigned surplus	115.9	103.1
Statutory surplus	143.5	128.7
Statutory net income	16.8	18.5	12.9
Selective Insurance Company of New York (SICNY) [Member]
Statutory unassigned surplus	111.7	99.4
Statutory surplus	139.4	127.1
Statutory net income	15.3	17.0	12.0
Selective Insurance Company of New England (SICNE) [Member]
Statutory unassigned surplus	30.0	25.3
Statutory surplus	61.2	55.4
Statutory net income	6.8	7.8	5.6
Selective Auto Insurance Company of New Jersey (SAICNJ) [Member]
Statutory unassigned surplus	70.0	62.5
Statutory surplus	114.9	105.4
Statutory net income	12.9	14.9	9.9
Mesa Underwriting Specialty Insurance Company (MUSIC) [Member]
Statutory unassigned surplus	34.4	27.1
Statutory surplus	103.9	95.6
Statutory net income	11.4	13.2	9.4
Selective Casualty Insurance Company (SCIC) [Member]
Statutory unassigned surplus	71.1	58.2
Statutory surplus	147.5	132.7
Statutory net income	16.2	16.8	13.3
Selective Fire and Casualty Insurance Company (SFCIC) [Member]
Statutory unassigned surplus	29.2	23.5
Statutory surplus	62.1	55.4
Statutory net income	$ 6.4	$ 7.5	$ 5.5

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

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(c))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24708-158529

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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(c))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

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