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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, 2021	Dec. 31, 2020	Dec. 31, 2019
Statutory unassigned surplus	$ 1,825.4	$ 1,559.1
Statutory surplus	2,391.0	2,124.7
Statutory net income	338.3	242.4	$ 292.7
Selective Insurace Company of America (SICA) [Member]
Statutory unassigned surplus	673.1	574.2
Statutory surplus	838.3	739.4
Statutory net income	134.7	81.8	113.9
Selective Way Insurance Company (SWIC) [Member]
Statutory unassigned surplus	436.4	374.0
Statutory surplus	492.4	430.0
Statutory net income	74.5	54.0	59.2
Selective Insurance Company of South Carolina (SICSC) [Member]
Statutory unassigned surplus	166.3	148.6
Statutory surplus	200.6	182.8
Statutory net income	24.2	20.8	23.9
Selective Insurance Company of the Southeast (SICSE) [Member]
Statutory unassigned surplus	132.7	115.9
Statutory surplus	160.3	143.5
Statutory net income	19.4	16.8	18.5
Selective Insurance Company of New York (SICNY) [Member]
Statutory unassigned surplus	127.0	111.7
Statutory surplus	154.7	139.4
Statutory net income	18.6	15.3	17.0
Selective Insurance Company of New England (SICNE) [Member]
Statutory unassigned surplus	34.5	30.0
Statutory surplus	65.6	61.2
Statutory net income	7.5	6.8	7.8
Selective Auto Insurance Company of New Jersey (SAICNJ) [Member]
Statutory unassigned surplus	90.4	70.0
Statutory surplus	135.2	114.9
Statutory net income	16.7	12.9	14.9
Mesa Underwriting Specialty Insurance Company (MUSIC) [Member]
Statutory unassigned surplus	47.4	34.4
Statutory surplus	116.9	103.9
Statutory net income	13.9	11.4	13.2
Selective Casualty Insurance Company (SCIC) [Member]
Statutory unassigned surplus	83.4	71.1
Statutory surplus	159.9	147.5
Statutory net income	20.6	16.2	16.8
Selective Fire and Casualty Insurance Company (SFCIC) [Member]
Statutory unassigned surplus	34.2	29.2
Statutory surplus	67.1	62.1
Statutory net income	$ 8.2	$ 6.4	$ 7.5

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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