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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010		Jun. 30, 2009
Fixed maturity securities, AFS	$ 2,897,373		$ 2,342,742
Equity securities, AFS	157,355		69,636		31,100
Short-term investments	217,044		161,155
Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	2,897,373		2,342,742
Equity securities, AFS	157,355		69,636
Short-term investments	217,044		161,155
Receivable for proceeds related to sale of Selective HR	3,212		5,002
Total financial assets measured on a recurring basis	3,274,984		2,578,535
Fair Value [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	353,796	[1]	320,529	[1]
Fair Value [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	34,173		18,966
Fair Value [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	622,659		533,897
Fair Value [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	1,213,310		993,725
Fair Value [Member] | Asset-backed Securities ("ABS") [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	78,949		48,733
Fair Value [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	112,598		104,475
Fair Value [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	481,888		322,417
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	126,475		105,317
Equity securities, AFS	157,355		69,636
Short-term investments	217,044		161,155
Receivable for proceeds related to sale of Selective HR	0		0
Total financial assets measured on a recurring basis	500,874		336,108
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	126,475	[1]	105,317	[1]
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Asset-backed Securities ("ABS") [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	2,746,200		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	2,746,200		2,237,240
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	205,580	[1]	215,212	[1]
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	34,173		18,966
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	622,659		533,897
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	1,210,707		993,725
Significant Other Observable Inputs (Level 2) [Member] | Asset-backed Securities ("ABS") [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	78,949		48,733
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	112,244		104,290
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	481,888		322,417
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	24,698		185
Equity securities, AFS	0		0
Short-term investments	0		0
Receivable for proceeds related to sale of Selective HR	3,212		5,002
Total financial assets measured on a recurring basis	27,910		5,187
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	21,741	[1]	0	[1]
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	2,603		0
Significant Unobservable Inputs (Level 3) [Member] | Asset-backed Securities ("ABS") [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	0		0
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	354		185
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value, Measurements, Recurring [Member]
Fixed maturity securities, AFS	$ 0		$ 0
[1] 1 U.S. government includes corporate securities fully guaranteed by the FDIC.

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