Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
AFS fixed maturity securities	$ 3,269,960		$ 2,897,373
Equity securities, AFS	155,577		157,355
Short-term investments	184,878		217,044
Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	3,269,960		2,897,373
Equity securities, AFS	155,577		157,355
Short-term investments	184,878		217,044
Receivable for proceeds related to sale of Selective HR	2,799		3,212
Total assets	3,613,214		3,274,984
Fair Value [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	294,214	[1]	353,796	[1]
Fair Value [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	30,292		34,173
Fair Value [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	784,501		622,659
Fair Value [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	1,425,888		1,213,310
Fair Value [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	121,154		78,949
Fair Value [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	129,615		112,598
Fair Value [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	484,296		481,888
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	130,435	[2]	126,475	[2]
Equity securities, AFS	151,970	[2]	157,355	[2]
Short-term investments	184,878	[2]	217,044	[2]
Receivable for proceeds related to sale of Selective HR	0	[2]	0	[2]
Total assets	467,283	[2]	500,874	[2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	130,435	[1],[2]	126,475	[1],[2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	0	[2]	0	[2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	0	[2]	0	[2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	0	[2]	0	[2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	0	[2]	0	[2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	0	[2]	0	[2]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	0	[2]	0	[2]
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	3,094,637	[2]	2,746,200	[2]
Equity securities, AFS	0	[2]	0	[2]
Short-term investments	0	[2]	0	[2]
Receivable for proceeds related to sale of Selective HR	0	[2]	0	[2]
Total assets	3,094,637	[2]	2,746,200	[2]
Significant Other Observable Inputs (Level 2) [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	143,804	[1],[2]	205,580	[1],[2]
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	30,292	[2]	34,173	[2]
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	784,501	[2]	622,659	[2]
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	1,423,005	[2]	1,210,707	[2]
Significant Other Observable Inputs (Level 2) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	113,835	[2]	78,949	[2]
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	114,904	[2]	112,244	[2]
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	484,296	[2]	481,888	[2]
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	44,888		24,698
Equity securities, AFS	3,607		0
Short-term investments	0		0
Receivable for proceeds related to sale of Selective HR	2,799		3,212
Total assets	51,294		27,910
Significant Unobservable Inputs (Level 3) [Member] | US Government and Government Agencies and Authorities [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	19,975	[1]	21,741	[1]
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	0		0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	0		0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	2,883		2,603
Significant Unobservable Inputs (Level 3) [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	7,319		0
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	14,711		354
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member]
AFS fixed maturity securities	$ 0		$ 0

[1] 2 U.S. government includes corporate securities fully guaranteed by the FDIC.

[2] 1 There were no transfers of securities between Level 1 and Level 2.