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Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified | Sep. 30, 2012 | Dec. 31, 2011
--- | --- | ---
Held-to-maturity Securities | $ 600,716 | $ 712,348
AFS fixed maturity securities | 3,269,960 | 2,897,373
Equity securities, AFS | 155,577 | 157,355
Short-term investments | 184,878 | 217,044
Notes payable | 307,380 | 307,360
Percentage of fixed maturity portfolio utilizing market approach | 95.00% |
Carrying Amount [Member] |  |
Held-to-maturity Securities | 600,716 | 712,348
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
Notes payable | 307,380 | 307,360
Carrying Amount [Member] | Seven Point Twenty Five Percent Senior Notes [Member] |  |
Notes payable | 49,911 | 49,908
Carrying Amount [Member] | Six Point Seventy Percent Senior Notes [Member] |  |
Notes payable | 99,469 | 99,452
Carrying Amount [Member] | Seven Point Fifty Percent Junior Notes [Member] |  |
Notes payable | 100,000 | 100,000
Carrying Amount [Member] | Two Point Ninety Percent Borrowing From Federal Home Loan Bank [Member] |  |
Notes payable | 13,000 | 13,000
Carrying Amount [Member] | One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member] |  |
Notes payable | 45,000 | 45,000
Fair Value [Member] |  |
Held-to-maturity Securities | 646,035 | 758,043
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
Notes payable | 323,900 | 323,054
Fair Value [Member] | Seven Point Twenty Five Percent Senior Notes [Member] |  |
Notes payable | 62,201 | 51,111
Fair Value [Member] | Six Point Seventy Percent Senior Notes [Member] |  |
Notes payable | 102,169 | 113,195
Fair Value [Member] | Seven Point Fifty Percent Junior Notes [Member] |  |
Notes payable | 100,200 | 100,360
Fair Value [Member] | Two Point Ninety Percent Borrowing From Federal Home Loan Bank [Member] |  |
Notes payable | 13,667 | 13,759
Fair Value [Member] | One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member] |  |
Notes payable | $ 45,663 | $ 44,629

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