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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) (USD $) In Thousands	Sep. 30, 2011		Dec. 31, 2010
Fixed maturity securities, AFS	$ 2,772,348		$ 2,342,742
Equity securities, AFS	139,203		69,636
Short-term investments	162,812		161,155
U.S. Government and Government Agencies [Member] | Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	380,031	[1]	320,529	[1]
Foreign Government [Member] | Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	32,912		18,966
Obligations of States and Political Subdivisions [Member] | Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	561,515		533,897
Corporate Securities [Member] | Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	1,125,559		993,725
Asset-backed Securities ("ABS") [Member] | Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	78,694		48,733
Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	112,891		104,475
Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	480,746		322,417
Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	2,772,348		2,342,742
Equity securities, AFS	139,203		69,636
Short-term investments	162,812		161,155
Receivable for proceeds related to sale of Selective HR	3,375		5,002
Total financial assets measured on a recurring basis	3,077,738		2,578,535
U.S. Government and Government Agencies [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	122,701	[1]	105,317	[1]
Foreign Government [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Obligations of States and Political Subdivisions [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Corporate Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Asset-backed Securities ("ABS") [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Commercial Mortgage-backed Securities ("CMBS") [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Residential Mortgage Backed Securities ("RMBS") [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	122,701		105,317
Equity securities, AFS	139,203		69,636
Short-term investments	162,812		161,155
Receivable for proceeds related to sale of Selective HR	0		0
Total financial assets measured on a recurring basis	424,716		336,108
U.S. Government and Government Agencies [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	257,330	[1]	215,212	[1]
Foreign Government [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	32,912		18,966
Obligations of States and Political Subdivisions [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	561,515		533,897
Corporate Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	1,125,559		993,725
Asset-backed Securities ("ABS") [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	78,694		48,733
Commercial Mortgage-backed Securities ("CMBS") [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	112,561		104,290
Residential Mortgage Backed Securities ("RMBS") [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	480,746		322,417
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	2,649,317		2,237,240
Equity securities, AFS	0		0
Short-term investments	0		0
Receivable for proceeds related to sale of Selective HR	0		0
Total financial assets measured on a recurring basis	2,649,317		2,237,240
U.S. Government and Government Agencies [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0	[1]	0	[1]
Foreign Government [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Obligations of States and Political Subdivisions [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Corporate Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Asset-backed Securities ("ABS") [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Commercial Mortgage-backed Securities ("CMBS") [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	330		185
Residential Mortgage Backed Securities ("RMBS") [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	330		185
Equity securities, AFS	0		0
Short-term investments	0		0
Receivable for proceeds related to sale of Selective HR	3,375		5,002
Total financial assets measured on a recurring basis	$ 3,705		$ 5,187
[1] 1 U.S. government includes corporate securities fully guaranteed by the FDIC.

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