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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, 2019	Dec. 31, 2018	Dec. 31, 2017
Statutory unassigned surplus	$ 1,396.8	$ 1,232.8
Statutory surplus	1,932.4	1,768.4
Statutory net income	292.7	210.6	$ 221.2
Selective Insurace Company of America (SICA) [Member]
Statutory unassigned surplus	525.9	478.6
Statutory surplus	680.1	632.8
Statutory net income	113.9	78.0	84.6
Selective Way Insurance Company (SWIC) [Member]
Statutory unassigned surplus	339.2	300.2
Statutory surplus	388.2	349.3
Statutory net income	59.2	47.5	43.6
Selective Insurance Company of South Carolina (SICSC) [Member]
Statutory unassigned surplus	132.6	119.4
Statutory surplus	163.8	150.7
Statutory net income	23.9	16.5	17.9
Selective Insurance Company of the Southeast (SICSE) [Member]
Statutory unassigned surplus	103.1	92.2
Statutory surplus	128.7	117.7
Statutory net income	18.5	12.9	14.7
Selective Insurance Company of New York (SICNY) [Member]
Statutory unassigned surplus	99.4	86.5
Statutory surplus	127.1	114.2
Statutory net income	17.0	12.0	13.4
Selective Insurance Company of New England (SICNE) [Member]
Statutory unassigned surplus	25.3	19.9
Statutory surplus	55.4	50.0
Statutory net income	7.8	5.6	6.3
Selective Auto Insurance Company of New Jersey (SAICNJ) [Member]
Statutory unassigned surplus	62.5	50.3
Statutory surplus	105.4	93.2
Statutory net income	14.9	9.9	11.4
Mesa Underwriting Specialty Insurance Company (MUSIC) [Member]
Statutory unassigned surplus	27.1	23.0
Statutory surplus	95.6	91.5
Statutory net income	13.2	9.4	10.3
Selective Casualty Insurance Company (SCIC) [Member]
Statutory unassigned surplus	58.2	44.9
Statutory surplus	132.7	119.3
Statutory net income	16.8	13.3	13.4
Selective Fire and Casualty Insurance Company (SFCIC) [Member]
Statutory unassigned surplus	23.5	17.8
Statutory surplus	55.4	49.7
Statutory net income	$ 7.5	$ 5.5	$ 5.6

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=119400593&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=119400593&loc=d3e572229-122910

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