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Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Held To Maturity Securities	$ 852,843	$ 1,214,324
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 Solutions	262,353	262,333
Carrying Amount [Member]
Held To Maturity Securities	852,843	1,214,324
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
Receivable for proceeds related to sale of Selective HR Solutions	262,353	262,333
Carrying Amount [Member] | Seven Point Twenty Five Percent Senior Notes [Member]
Receivable for proceeds related to sale of Selective HR Solutions	49,907	49,904
Carrying Amount [Member] | Six Point Seventy Percent Senior Notes [Member]
Receivable for proceeds related to sale of Selective HR Solutions	99,446	99,429
Carrying Amount [Member] | Seven Point Fifty Percent Junior Notes [Member]
Receivable for proceeds related to sale of Selective HR Solutions	100,000	100,000
Carrying Amount [Member] | Two Point Ninety Percent Borrowings from FHLBI [Member]
Receivable for proceeds related to sale of Selective HR Solutions	13,000	13,000
Fair Value [Member]
Held To Maturity Securities	901,385	1,256,294
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
Receivable for proceeds related to sale of Selective HR Solutions	281,637	258,516
Fair Value [Member] | Seven Point Twenty Five Percent Senior Notes [Member]
Receivable for proceeds related to sale of Selective HR Solutions	58,589	55,190
Fair Value [Member] | Six Point Seventy Percent Senior Notes [Member]
Receivable for proceeds related to sale of Selective HR Solutions	110,604	90,097
Fair Value [Member] | Seven Point Fifty Percent Junior Notes [Member]
Receivable for proceeds related to sale of Selective HR Solutions	98,664	99,840
Fair Value [Member] | Two Point Ninety Percent Borrowings from FHLBI [Member]
Receivable for proceeds related to sale of Selective HR Solutions	$ 13,780	$ 13,389

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