Statutory Financial Information, Capital Requirements and Restrictions on Dividends and Transfers of Funds (Statutory Financial Information) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statutory unassigned surplus	$ 772.2	$ 720.8
Statutory surplus	1,307.8	1,256.4
Statutory net income	190.3	122.2	50.4
Selective Insurace Company of America (SICA) [Member]
Statutory unassigned surplus	338.8	309.2
Statutory surplus	493.0	463.4
Statutory net income	83.9	53.1	29.8
Selective Way Insurance Company (SWIC) [Member]
Statutory unassigned surplus	201.3	201.3
Statutory surplus	250.3	250.3
Statutory net income	37.0	27.5	10.1
Selective Insurance Company of South Carolina (SICSC) [Member]
Statutory unassigned surplus	83.9	80.7
Statutory surplus	115.1	111.9
Statutory net income	14.0	8.2	2.8
Selective Insurance Company of the Southeast (SICSE) [Member]
Statutory unassigned surplus	59.3	56.2
Statutory surplus	84.9	81.8
Statutory net income	10.5	6.0	1.6
Selective Insurance Company of New York (SICNY) [Member]
Statutory unassigned surplus	54.9	51.5
Statutory surplus	82.6	79.3
Statutory net income	10.3	6.9	2.7
Selective Insurance Company of New England (SICNE) [Member]
Statutory unassigned surplus	5.3	4.7
Statutory surplus	35.4	34.9
Statutory net income	4.4	3.1	0.6
Selective Auto Insurance Company of New Jersey (SAICNJ) [Member]
Statutory unassigned surplus	18.4	14.2
Statutory surplus	61.3	57.0
Statutory net income	9.1	2.5	1.5
Mesa Underwriting Specialty Insurance Company (MUSIC) [Member]
Statutory unassigned surplus	(1.7)	(6.2)
Statutory surplus	66.8	62.3
Statutory net income	7.3	5.2	0.9
Selective Casualty Insurance Company (SCIC) [Member]
Statutory unassigned surplus	8.2	6.1
Statutory surplus	82.7	80.5
Statutory net income	9.6	6.6	0.2
Selective Fire and Casualty Insurance Company (SFCIC) [Member]
Statutory unassigned surplus	3.8	3.1
Statutory surplus	35.7	35.0
Statutory net income	$ 4.2	$ 3.1	$ 0.2