Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Fair Value, Assets Measured On Recurring Basis, Arbitrage Trading Account assets	$ 242,720	$ 339,235
Fair Value Assets Measured On Recurring Basis Total Assets	12,149,116	12,013,404
Fair Value, Liabilities Measured On Recurring Basis, Securities Sold, Not Yet Purchased	68,255	53,494
Common Stocks [Member] | Equity Securities Available For Sale [Member]
Total Equity Securities Available For Sale	348,259	316,056
Common Stocks [Member] | Equity Securities Available For Sale [Member] | Level 1 [Member]
Total Equity Securities Available For Sale	290,956	204,749
Common Stocks [Member] | Equity Securities Available For Sale [Member] | Level 2 [Member]
Total Equity Securities Available For Sale	55,744	109,748
Common Stocks [Member] | Equity Securities Available For Sale [Member] | Level 3 [Member]
Total Equity Securities Available For Sale	1,559	1,559
Preferred Stocks [Member] | Equity Securities Available For Sale [Member]
Total Equity Securities Available For Sale	162,793	244,997
Preferred Stocks [Member] | Equity Securities Available For Sale [Member] | Level 2 [Member]
Total Equity Securities Available For Sale	136,594	155,551
Preferred Stocks [Member] | Equity Securities Available For Sale [Member] | Level 3 [Member]
Total Equity Securities Available For Sale	26,199	89,446
Equity Securities Available For Sale [Member]
Total Equity Securities Available For Sale	511,052	561,053
Equity Securities Available For Sale [Member] | Level 1 [Member]
Total Equity Securities Available For Sale	290,956	204,749
Equity Securities Available For Sale [Member] | Level 2 [Member]
Total Equity Securities Available For Sale	192,338	265,299
Equity Securities Available For Sale [Member] | Level 3 [Member]
Total Equity Securities Available For Sale	27,758	91,005
U.S. Government And Government Agency [Member] | Fixed Maturities Securities [Member]
Total Equity Securities Available For Sale	1,177,289	1,347,875
U.S. Government And Government Agency [Member] | Fixed Maturities Securities [Member] | Level 2 [Member]
Total Equity Securities Available For Sale	1,177,289	1,347,875
U.S. Government And Government Agency [Member] | Fixed Maturities Securities [Member] | Level 3 [Member]
Total Equity Securities Available For Sale
State And Municipal [Member] | Fixed Maturities Securities [Member]
Total Equity Securities Available For Sale	5,386,614	5,461,446
State And Municipal [Member] | Fixed Maturities Securities [Member] | Level 2 [Member]
Total Equity Securities Available For Sale	5,386,614	5,461,446
State And Municipal [Member] | Fixed Maturities Securities [Member] | Level 3 [Member]
Total Equity Securities Available For Sale
Mortgage-Backed Securities [Member] | Fixed Maturities Securities [Member]
Total Equity Securities Available For Sale	1,439,587	1,411,846
Mortgage-Backed Securities [Member] | Fixed Maturities Securities [Member] | Level 2 [Member]
Total Equity Securities Available For Sale	1,439,587	1,411,846
Mortgage-Backed Securities [Member] | Fixed Maturities Securities [Member] | Level 3 [Member]
Total Equity Securities Available For Sale
Corporate [Member] | Fixed Maturities Securities [Member]
Total Equity Securities Available For Sale	2,609,285	2,380,262
Corporate [Member] | Fixed Maturities Securities [Member] | Level 2 [Member]
Total Equity Securities Available For Sale	2,534,716	2,292,199
Corporate [Member] | Fixed Maturities Securities [Member] | Level 3 [Member]
Total Equity Securities Available For Sale	74,569	88,063
Foreign [Member] | Fixed Maturities Securities [Member]
Total Equity Securities Available For Sale	550,502	491,730
Foreign [Member] | Fixed Maturities Securities [Member] | Level 2 [Member]
Total Equity Securities Available For Sale	550,502	491,730
Foreign [Member] | Fixed Maturities Securities [Member] | Level 3 [Member]
Total Equity Securities Available For Sale
Fixed Maturities Securities [Member]
Total Equity Securities Available For Sale	11,163,277	11,093,159
Fixed Maturities Securities [Member] | Level 2 [Member]
Total Equity Securities Available For Sale	11,088,708	11,005,096
Fixed Maturities Securities [Member] | Level 3 [Member]
Total Equity Securities Available For Sale	74,569	88,063
Arbitrage Trading Account [Member]
Fair Value, Assets Measured On Recurring Basis, Arbitrage Trading Account assets	474,787	359,192
Arbitrage Trading Account [Member] | Level 1 [Member]
Fair Value, Assets Measured On Recurring Basis, Arbitrage Trading Account assets	231,120	162,292
Arbitrage Trading Account [Member] | Level 2 [Member]
Fair Value, Assets Measured On Recurring Basis, Arbitrage Trading Account assets	242,569	193,713
Arbitrage Trading Account [Member] | Level 3 [Member]
Fair Value, Assets Measured On Recurring Basis, Arbitrage Trading Account assets	1,098	3,187
Level 1 [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	522,076	367,041
Fair Value, Liabilities Measured On Recurring Basis, Securities Sold, Not Yet Purchased	48,759	51,672
Level 2 [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	11,523,615	11,464,108
Fair Value, Liabilities Measured On Recurring Basis, Securities Sold, Not Yet Purchased	19,495	1,822
Level 3 [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	103,425	182,255
Fair Value, Liabilities Measured On Recurring Basis, Securities Sold, Not Yet Purchased	$ 1