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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, 2016	Dec. 31, 2015	Dec. 31, 2014
Statutory unassigned surplus	$ 1,048.2	$ 890.7
Statutory surplus	1,583.8	1,426.3
Statutory net income	202.3	195.8	$ 190.3
Selective Insurace Company of America (SICA) [Member]
Statutory unassigned surplus	414.4	366.6
Statutory surplus	568.6	520.8
Statutory net income	72.2	69.6	83.9
Selective Way Insurance Company (SWIC) [Member]
Statutory unassigned surplus	260.5	223.6
Statutory surplus	309.5	272.6
Statutory net income	41.2	42.3	37.0
Selective Insurance Company of South Carolina (SICSC) [Member]
Statutory unassigned surplus	110.6	96.6
Statutory surplus	141.9	127.9
Statutory net income	17.4	15.9	14.0
Selective Insurance Company of the Southeast (SICSE) [Member]
Statutory unassigned surplus	83.5	70.7
Statutory surplus	109.1	96.2
Statutory net income	13.4	12.1	10.5
Selective Insurance Company of New York (SICNY) [Member]
Statutory unassigned surplus	74.1	65.3
Statutory surplus	101.8	93.0
Statutory net income	12.9	12.7	10.3
Selective Insurance Company of New England (SICNE) [Member]
Statutory unassigned surplus	13.6	9.2
Statutory surplus	43.7	39.4
Statutory net income	5.9	5.5	4.4
Selective Auto Insurance Company of New Jersey (SAICNJ) [Member]
Statutory unassigned surplus	36.9	26.4
Statutory surplus	79.8	69.2
Statutory net income	11.5	10.8	9.1
Mesa Underwriting Specialty Insurance Company (MUSIC) [Member]
Statutory unassigned surplus	16.7	7.0
Statutory surplus	85.2	75.5
Statutory net income	9.7	9.5	7.3
Selective Casualty Insurance Company (SCIC) [Member]
Statutory unassigned surplus	26.6	17.8
Statutory surplus	101.0	92.3
Statutory net income	12.6	12.1	9.6
Selective Fire and Casualty Insurance Company (SFCIC) [Member]
Statutory unassigned surplus	11.3	7.5
Statutory surplus	43.2	39.4
Statutory net income	$ 5.5	$ 5.3	$ 4.2

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.

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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/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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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