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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, 2018	Dec. 31, 2017	Dec. 31, 2016
Statutory unassigned surplus	$ 1,232.8	$ 1,137.3
Statutory surplus	1,768.4	1,672.9
Statutory net income	210.6	221.2	$ 202.3
Selective Insurace Company of America (SICA) [Member]
Statutory unassigned surplus	478.6	455.5
Statutory surplus	632.8	609.7
Statutory net income	78.0	84.6	72.2
Selective Way Insurance Company (SWIC) [Member]
Statutory unassigned surplus	300.2	276.1
Statutory surplus	349.3	325.1
Statutory net income	47.5	43.6	41.2
Selective Insurance Company of South Carolina (SICSC) [Member]
Statutory unassigned surplus	119.4	112.9
Statutory surplus	150.7	144.1
Statutory net income	16.5	17.9	17.4
Selective Insurance Company of the Southeast (SICSE) [Member]
Statutory unassigned surplus	92.2	86.2
Statutory surplus	117.7	111.8
Statutory net income	12.9	14.7	13.4
Selective Insurance Company of New York (SICNY) [Member]
Statutory unassigned surplus	86.5	78.8
Statutory surplus	114.2	106.5
Statutory net income	12.0	13.4	12.9
Selective Insurance Company of New England (SICNE) [Member]
Statutory unassigned surplus	19.9	16.1
Statutory surplus	50.0	46.3
Statutory net income	5.6	6.3	5.9
Selective Auto Insurance Company of New Jersey (SAICNJ) [Member]
Statutory unassigned surplus	50.3	42.1
Statutory surplus	93.2	84.9
Statutory net income	9.9	11.4	11.5
Mesa Underwriting Specialty Insurance Company (MUSIC) [Member]
Statutory unassigned surplus	23.0	21.4
Statutory surplus	91.5	89.9
Statutory net income	9.4	10.3	9.7
Selective Casualty Insurance Company (SCIC) [Member]
Statutory unassigned surplus	44.9	34.5
Statutory surplus	119.3	109.0
Statutory net income	13.3	13.4	12.6
Selective Fire and Casualty Insurance Company (SFCIC) [Member]
Statutory unassigned surplus	17.8	13.7
Statutory surplus	49.7	45.6
Statutory net income	$ 5.5	$ 5.6	$ 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=6879938&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=6879938&loc=d3e572229-122910

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