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, 2013	Dec. 31, 2012	Dec. 31, 2011
Statutory unassigned surplus	$ 720.8	$ 571.6
Statutory surplus	1,256.4	1,050.1
Statutory net income	122.2	50.4	26.5
Selective Insurace Company of America (SICA) [Member]
Statutory unassigned surplus	309.2	246.2
Statutory surplus	463.4	369.9
Statutory net income	53.1	29.8	15.2
Selective Way Insurance Company (SWIC) [Member]
Statutory unassigned surplus	201.3	167.6
Statutory surplus	250.3	211.2
Statutory net income	27.5	10.1	7.8
Selective Insurance Company of South Carolina (SICSC) [Member]
Statutory unassigned surplus	80.7	70.1
Statutory surplus	111.9	91.4
Statutory net income	8.2	2.8	0.7
Selective Insurance Company of the Southeast (SICSE) [Member]
Statutory unassigned surplus	56.2	50.1
Statutory surplus	81.8	69.7
Statutory net income	6.0	1.6	0.3
Selective Insurance Company of New York (SICNY) [Member]
Statutory unassigned surplus	51.5	45.3
Statutory surplus	79.3	72.6
Statutory net income	6.9	2.7	1.5
Selective Insurance Company of New England (SICNE) [Member]
Statutory unassigned surplus	4.7	2.7
Statutory surplus	34.9	32.5
Statutory net income	3.1	0.6	0.3
Selective Auto Insurance Company of New Jersey (SAICNJ) [Member]
Statutory unassigned surplus	14.2	7.6
Statutory surplus	57.0	45.9
Statutory net income	2.5	1.5	0.7
Mesa Underwriting Specialty Insurance Company (MUSIC) [Member]
Statutory unassigned surplus	(6.2)	(14.9)
Statutory surplus	62.3	53.6
Statutory net income	5.2	0.9	0
Selective Casualty Insurance Company (SCIC) [Member]
Statutory unassigned surplus	6.1	(2.2)
Statutory surplus	80.5	72.2
Statutory net income	6.6	0.2	0
Selective Fire and Casualty Insurance Company (SFCIC) [Member]
Statutory unassigned surplus	3.1	(0.9)
Statutory surplus	35.0	31.1
Statutory net income	$ 3.1	$ 0.2	$ 0