Statutory Financial Information and Restrictions on Dividends and Transfers of Funds (Statutory Surplus) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Statutory Accounting Practices [Line Items]
Unassigned surplus	$ 610.0		$ 679.1
Statutory surplus	1,100.9		1,076.5
Statutory net income	36.3	2.6
Additional Borrowing Capacity for Parent	18.0
Selective Insurance Company of America [Member]
Statutory Accounting Practices [Line Items]
Unassigned surplus	261.9		360.7
Statutory surplus	385.6		507.4
Statutory net income	47.4	5.1
Selective Way Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Unassigned surplus	179.6		169.1
Statutory surplus	223.2		221.7
Statutory net income	7.1	2.8
Selective Insurance Company of South Carolina [Member]
Statutory Accounting Practices [Line Items]
Unassigned surplus	72.2		65.5
Statutory surplus	93.4		90.5
Statutory net income	2.6	(1.9)
Selective Insurance Company of the Southeast [Member]
Statutory Accounting Practices [Line Items]
Unassigned surplus	51.6		46.8
Statutory surplus	71.2		69.3
Statutory net income	1.6	(1.5)
Selective Insurance Company of New York [Member]
Statutory Accounting Practices [Line Items]
Unassigned surplus	47.4		43.0
Statutory surplus	74.7		73.3
Statutory net income	2.1	(1.0)
Selective Insurance Company of New England [Member]
Statutory Accounting Practices [Line Items]
Unassigned surplus	2.9		3.8
Statutory surplus	32.7		14.3
Statutory net income	(3.6)	0.2
Selective Auto Insurance Company of New Jersey [Member]
Statutory Accounting Practices [Line Items]
Unassigned surplus	9.1		5.6
Statutory surplus	59.9		60.1
Statutory net income	1.6	(1.1)
Mesa Underwriting Specialty Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Unassigned surplus	(12.5)		(15.4)
Statutory surplus	56.0		39.9
Statutory net income	(6.5)
Selective Casualty Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Unassigned surplus	(1.6)
Statutory surplus	72.9
Statutory net income	(11.2)
Selective Fire and Casualty Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Unassigned surplus	(0.6)
Statutory surplus	31.3
Statutory net income	$ (4.8)