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Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Held-to-maturity Securities | $ 507,771 | $ 554,069
Available-for-sale Secuities | 3,465,981 | 3,296,013
Equity securities, AFS | 174,745 | 151,382
Short-term investments | 163,440 | 214,479
Notes payable | 392,393 | 307,387
Carrying Amount [Member] |  |
Held-to-maturity Securities | 507,771 | 554,069
Available-for-sale Secuities | 3,465,981 | 3,296,013
Equity securities, AFS | 174,745 | 151,382
Short-term investments | 163,440 | 214,479
Receivable for proceeds related to sale of Selective HR | 0 | 2,705
Notes payable | 392,393 | 307,387
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
Carrying Amount [Member] | Seven Point Fifty Percent Junior Notes [Member] |  |
Notes payable | 0 | 100,000
Carrying Amount [Member] | Six Point Seventy Percent Senior Notes [Member] |  |
Notes payable | 99,481 | 99,475
Carrying Amount [Member] | Seven Point Twenty Five Percent Senior Notes [Member] |  |
Notes payable | 49,912 | 49,912
Carrying Amount [Member] | Five Point Eight Hundred Seventy Five Senior Notes [Member] |  |
Notes payable | 185,000 | 0
Fair Value [Member] |  |
Held-to-maturity Securities | 543,553 | 594,661
Available-for-sale Secuities | 3,465,981 | 3,296,013
Equity securities, AFS | 174,745 | 151,382
Short-term investments | 163,440 | 214,479
Receivable for proceeds related to sale of Selective HR | 0 | 2,705
Notes payable | 407,274 | 321,061
Fair Value [Member] | Two Point Ninety Percent Borrowing From Federal Home Loan Bank [Member] |  |
Notes payable | 13,539 | 13,595
Fair Value [Member] | One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member] |  |
Notes payable | 45,649 | 45,590
Fair Value [Member] | Seven Point Fifty Percent Junior Notes [Member] |  |
Notes payable | 0 | 101,480
Fair Value [Member] | Six Point Seventy Percent Senior Notes [Member] |  |
Notes payable | 110,815 | 107,707
Fair Value [Member] | Seven Point Twenty Five Percent Senior Notes [Member] |  |
Notes payable | 52,641 | 52,689
Fair Value [Member] | Five Point Eight Hundred Seventy Five Senior Notes [Member] |  |
Notes payable | $ 184,630 | $ 0

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