Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value Assets Measured On Recurring Basis Total Assets	$ 12,691,714	$ 12,037,679
Fair Value, Assets Measured On Recurring Basis, Arbitrage Trading Account assets	275,844	318,240
Fair Value, Liabilities Measured On Recurring Basis, Securities Sold, Not Yet Purchased	40,520	62,514
Fixed Maturities Securities Available for Sale [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	11,710,680	11,196,928
Fixed Maturities Securities Available for Sale [Member] \| U.S. Government And Government Agency [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	985,544	976,493
Fixed Maturities Securities Available for Sale [Member] \| State And Municipal [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	5,154,709	5,323,070
Fixed Maturities Securities Available for Sale [Member] \| Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	1,630,968	1,584,587
Fixed Maturities Securities Available for Sale [Member] \| Corporate [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	2,912,788	2,424,424
Fixed Maturities Securities Available for Sale [Member] \| Foreign [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	1,026,671	888,354
Equity Securities Available For Sale [Member]
Total Equity Securities Available For Sale	561,785	443,439
Equity Securities Available For Sale [Member] \| Common Stock [Member]
Total Equity Securities Available For Sale	439,246	319,982
Equity Securities Available For Sale [Member] \| Preferred Stock [Member]
Total Equity Securities Available For Sale	122,539	123,457
Arbitrage Trading Account [Member]
Fair Value, Assets Measured On Recurring Basis, Arbitrage Trading Account assets	419,249	397,312
Level 1 [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	738,417	526,939
Fair Value, Liabilities Measured On Recurring Basis, Securities Sold, Not Yet Purchased	39,411	62,493
Level 1 [Member] \| Fixed Maturities Securities Available for Sale [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 1 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| U.S. Government And Government Agency [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 1 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| State And Municipal [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 1 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 1 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| Corporate [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 1 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| Foreign [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 1 [Member] \| Equity Securities Available For Sale [Member]
Total Equity Securities Available For Sale	437,837	318,423
Level 1 [Member] \| Equity Securities Available For Sale [Member] \| Common Stock [Member]
Total Equity Securities Available For Sale	437,837	318,423
Level 1 [Member] \| Equity Securities Available For Sale [Member] \| Preferred Stock [Member]
Total Equity Securities Available For Sale	0	0
Level 1 [Member] \| Arbitrage Trading Account [Member]
Fair Value, Assets Measured On Recurring Basis, Arbitrage Trading Account assets	300,580	208,516
Level 2 [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	11,881,966	11,428,199
Fair Value, Liabilities Measured On Recurring Basis, Securities Sold, Not Yet Purchased	1,089	0
Level 2 [Member] \| Fixed Maturities Securities Available for Sale [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	11,644,387	11,129,100
Level 2 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| U.S. Government And Government Agency [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	985,544	976,493
Level 2 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| State And Municipal [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	5,154,709	5,323,070
Level 2 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	1,630,968	1,584,587
Level 2 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| Corporate [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	2,846,495	2,356,596
Level 2 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| Foreign [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	1,026,671	888,354
Level 2 [Member] \| Equity Securities Available For Sale [Member]
Total Equity Securities Available For Sale	119,695	111,154
Level 2 [Member] \| Equity Securities Available For Sale [Member] \| Common Stock [Member]
Total Equity Securities Available For Sale	0	0
Level 2 [Member] \| Equity Securities Available For Sale [Member] \| Preferred Stock [Member]
Total Equity Securities Available For Sale	119,695	111,154
Level 2 [Member] \| Arbitrage Trading Account [Member]
Fair Value, Assets Measured On Recurring Basis, Arbitrage Trading Account assets	117,884	187,945
Level 3 [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	71,331	82,541
Fair Value, Liabilities Measured On Recurring Basis, Securities Sold, Not Yet Purchased	20	21
Level 3 [Member] \| Fixed Maturities Securities Available for Sale [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	66,293	67,828
Level 3 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| U.S. Government And Government Agency [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 3 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| State And Municipal [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 3 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 3 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| Corporate [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	66,293	67,828
Level 3 [Member] \| Fixed Maturities Securities Available for Sale [Member] \| Foreign [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 3 [Member] \| Equity Securities Available For Sale [Member]
Total Equity Securities Available For Sale	4,253	13,862
Level 3 [Member] \| Equity Securities Available For Sale [Member] \| Common Stock [Member]
Total Equity Securities Available For Sale	1,409	1,559
Level 3 [Member] \| Equity Securities Available For Sale [Member] \| Preferred Stock [Member]
Total Equity Securities Available For Sale	2,844	12,303
Level 3 [Member] \| Arbitrage Trading Account [Member]
Fair Value, Assets Measured On Recurring Basis, Arbitrage Trading Account assets	$ 785	$ 851

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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