Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Arbitrage trading account	$ 318,240	$ 339,235
Fixed maturites securities available for sale	12,037,679	12,013,404
Securities sold but not yet purchased	62,514	53,494
Level 1 [Member]
Fixed maturites securities available for sale	526,939	367,041
Securities sold but not yet purchased	62,493	51,672
Level 2 [Member]
Fixed maturites securities available for sale	11,428,199	11,464,108
Securities sold but not yet purchased	0	1,822
Level 3 [Member]
Fixed maturites securities available for sale	82,541	182,255
Securities sold but not yet purchased	21	0
Fixed Maturities Securities [Member]
Fixed maturites securities available for sale	11,196,928	11,093,159
Fixed Maturities Securities [Member] \| U.S. Government And Government Agency [Member]
Fixed maturites securities available for sale	976,493	1,347,875
Fixed Maturities Securities [Member] \| State and Municipal [Member]
Fixed maturites securities available for sale	5,323,070	5,461,446
Fixed Maturities Securities [Member] \| Mortgage-Backed Securities [Member]
Fixed maturites securities available for sale	1,584,587	1,411,846
Fixed Maturities Securities [Member] \| Corporate [Member]
Fixed maturites securities available for sale	2,424,424	2,119,922
Fixed Maturities Securities [Member] \| Foreign [Member]
Fixed maturites securities available for sale	888,354	752,070
Fixed Maturities Securities [Member] \| Level 1 [Member]
Fixed maturites securities available for sale	0	0
Fixed Maturities Securities [Member] \| Level 1 [Member] \| U.S. Government And Government Agency [Member]
Fixed maturites securities available for sale	0	0
Fixed Maturities Securities [Member] \| Level 1 [Member] \| State and Municipal [Member]
Fixed maturites securities available for sale	0	0
Fixed Maturities Securities [Member] \| Level 1 [Member] \| Mortgage-Backed Securities [Member]
Fixed maturites securities available for sale	0	0
Fixed Maturities Securities [Member] \| Level 1 [Member] \| Corporate [Member]
Fixed maturites securities available for sale	0	0
Fixed Maturities Securities [Member] \| Level 1 [Member] \| Foreign [Member]
Fixed maturites securities available for sale	0	0
Fixed Maturities Securities [Member] \| Level 2 [Member]
Fixed maturites securities available for sale	11,129,100	11,005,096
Fixed Maturities Securities [Member] \| Level 2 [Member] \| U.S. Government And Government Agency [Member]
Fixed maturites securities available for sale	976,493	1,347,875
Fixed Maturities Securities [Member] \| Level 2 [Member] \| State and Municipal [Member]
Fixed maturites securities available for sale	5,323,070	5,461,446
Fixed Maturities Securities [Member] \| Level 2 [Member] \| Mortgage-Backed Securities [Member]
Fixed maturites securities available for sale	1,584,587	1,411,846
Fixed Maturities Securities [Member] \| Level 2 [Member] \| Corporate [Member]
Fixed maturites securities available for sale	2,356,596	2,031,859
Fixed Maturities Securities [Member] \| Level 2 [Member] \| Foreign [Member]
Fixed maturites securities available for sale	888,354	752,070
Fixed Maturities Securities [Member] \| Level 3 [Member]
Fixed maturites securities available for sale	67,828	88,063
Fixed Maturities Securities [Member] \| Level 3 [Member] \| U.S. Government And Government Agency [Member]
Fixed maturites securities available for sale	0	0
Fixed Maturities Securities [Member] \| Level 3 [Member] \| State and Municipal [Member]
Fixed maturites securities available for sale	0	0
Fixed Maturities Securities [Member] \| Level 3 [Member] \| Mortgage-Backed Securities [Member]
Fixed maturites securities available for sale	0	0
Fixed Maturities Securities [Member] \| Level 3 [Member] \| Corporate [Member]
Fixed maturites securities available for sale	67,828	88,063
Fixed Maturities Securities [Member] \| Level 3 [Member] \| Foreign [Member]
Fixed maturites securities available for sale	0	0
Equity Securities Available For Sale [Member]
Total equity securities available for sale	443,439	561,053
Equity Securities Available For Sale [Member] \| Common Stocks [Member]
Total equity securities available for sale	319,982	316,056
Equity Securities Available For Sale [Member] \| Preferred Stocks [Member]
Total equity securities available for sale	123,457	244,997
Equity Securities Available For Sale [Member] \| Level 1 [Member]
Total equity securities available for sale	318,423	204,749
Equity Securities Available For Sale [Member] \| Level 1 [Member] \| Common Stocks [Member]
Total equity securities available for sale	318,423	204,749
Equity Securities Available For Sale [Member] \| Level 1 [Member] \| Preferred Stocks [Member]
Total equity securities available for sale	0	0
Equity Securities Available For Sale [Member] \| Level 2 [Member]
Total equity securities available for sale	111,154	265,299
Equity Securities Available For Sale [Member] \| Level 2 [Member] \| Common Stocks [Member]
Total equity securities available for sale	0	109,748
Equity Securities Available For Sale [Member] \| Level 2 [Member] \| Preferred Stocks [Member]
Total equity securities available for sale	111,154	155,551
Equity Securities Available For Sale [Member] \| Level 3 [Member]
Total equity securities available for sale	13,862	91,005
Equity Securities Available For Sale [Member] \| Level 3 [Member] \| Common Stocks [Member]
Total equity securities available for sale	1,559	1,559
Equity Securities Available For Sale [Member] \| Level 3 [Member] \| Preferred Stocks [Member]
Total equity securities available for sale	12,303	89,446
Arbitrage Trading Account [Member]
Arbitrage trading account	397,312	359,192
Arbitrage Trading Account [Member] \| Level 1 [Member]
Arbitrage trading account	208,516	162,292
Arbitrage Trading Account [Member] \| Level 2 [Member]
Arbitrage trading account	187,945	193,713
Arbitrage Trading Account [Member] \| Level 3 [Member]
Arbitrage trading account	$ 851	$ 3,187

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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