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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) (USD $) In Thousands	Jun. 30, 2011		Dec. 31, 2010
Fixed maturity securities, AFS	$ 2,525,237		$ 2,342,742
Equity securities, AFS	136,577		69,636
Short-term investments	142,809		161,155
U.S. Government and Government Agencies [Member] | Fair Value [Member] | Fair Value Measurements Recurring [Member]
Fixed maturity securities, AFS	296,931	[1]	320,529	[1]
Foreign Government [Member] | Fair Value [Member] | Fair Value Measurements Recurring [Member]
Fixed maturity securities, AFS	30,100		18,966
Obligations of States and Political Subdivisions [Member] | Fair Value [Member] | Fair Value Measurements Recurring [Member]
Fixed maturity securities, AFS	542,616		533,897
Corporate Securities [Member] | Fair Value [Member] | Fair Value Measurements Recurring [Member]
Fixed maturity securities, AFS	1,078,503		993,725
Asset-backed Securities ("ABS") [Member] | Fair Value [Member] | Fair Value Measurements Recurring [Member]
Fixed maturity securities, AFS	78,205		48,733
Commercial Mortgage-backed Securities ("CMBS") [Member] | Fair Value [Member] | Fair Value Measurements Recurring [Member]
Fixed maturity securities, AFS	109,124		104,475
Residential Mortgage Backed Securities ("RMBS") [Member] | Fair Value [Member] | Fair Value Measurements Recurring [Member]
Fixed maturity securities, AFS	389,758		322,417
Fair Value [Member] | Fair Value Measurements Recurring [Member]
Fixed maturity securities, AFS	2,525,237		2,342,742
Equity securities, AFS	136,577		69,636
Short-term investments	142,809		161,155
Receivable for proceeds related to sale of Selective HR Solutions ("Selective HR")	4,531		5,002
Total financial assets measured on a recurring basis	2,809,154		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	99,101	[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	99,101		105,317
Equity securities, AFS	136,577		69,636
Short-term investments	142,809		161,155
Receivable for proceeds related to sale of Selective HR Solutions ("Selective HR")	0		0
Total financial assets measured on a recurring basis	378,487		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	197,830	[1]	215,212	[1]
Foreign Government [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value Measurements Recurring [Member]
Fixed maturity securities, AFS	30,100		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	542,616		533,897
Corporate Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value Measurements Recurring [Member]
Fixed maturity securities, AFS	1,078,503		993,725
Asset-backed Securities ("ABS") [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value Measurements Recurring [Member]
Fixed maturity securities, AFS	78,205		48,733
Commercial Mortgage-backed Securities ("CMBS") [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value Measurements Recurring [Member]
Fixed maturity securities, AFS	108,683		104,290
Residential Mortgage Backed Securities ("RMBS") [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value Measurements Recurring [Member]
Fixed maturity securities, AFS	389,758		322,417
Significant Other Observable Inputs (Level 2) [Member] | Fair Value Measurements Recurring [Member]
Fixed maturity securities, AFS	2,425,695		2,237,240
Equity securities, AFS	0		0
Short-term investments	0		0
Receivable for proceeds related to sale of Selective HR Solutions ("Selective HR")	0		0
Total financial assets measured on a recurring basis	2,425,695		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	441		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	441		185
Equity securities, AFS	0		0
Short-term investments	0		0
Receivable for proceeds related to sale of Selective HR Solutions ("Selective HR")	4,531		5,002
Total financial assets measured on a recurring basis	$ 4,972		$ 5,187
[1] 1 U.S. government includes corporate securities fully guaranteed by the FDIC.

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- Definition

This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

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For an unclassified balance sheet, total of debt securities categorized neither as held-to-maturity nor trading. Such securities are reported at fair value; unrealized gains and losses of such securities are excluded from earnings and included in other comprehensive income, a separate component of shareholders' equity, unless the Available-for-sale Security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain or loss of an Available-for-sale Security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge, as should other than temporary declines in fair value below costs basis.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 13, 137

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 12
-Subparagraph b

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 16

Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Name Statement of Financial Accounting Standard (FAS)
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 133
-Paragraph 22

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 19

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 12
-Subparagraph b

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Emerging Issues Task Force (EITF)
-Number 86-40

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 13

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 16

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 3
-Subparagraph c

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher OTS
-Name Federal Regulation (FR)
-Number Title 12
-Chapter V
-Section 563c.102
-Subsection I

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 1
-Subparagraph g
-Article 7

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
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