Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value Assets Measured On Recurring Basis Total Assets	$ 12,663,423	$ 12,546,104
Fair Value, Assets Measured On Recurring Basis, Arbitrage Trading Account assets	28,720	446,873
Fair Value, Liabilities Measured On Recurring Basis, Securities Sold, Not Yet Purchased	79,965	121,487
Fixed Maturities Securities [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	11,562,814	11,841,005
Fixed Maturities Securities [Member] | U.S. Government And Government Agency [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	875,712	898,463
Fixed Maturities Securities [Member] | State And Municipal [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	4,409,616	4,768,519
Fixed Maturities Securities [Member] | Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	1,544,913	1,715,697
Fixed Maturities Securities [Member] | Corporate [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	3,571,079	3,346,719
Fixed Maturities Securities [Member] | Foreign [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	1,161,494	1,111,607
Equity Securities Available For Sale [Member]
Total Equity Securities Available For Sale	399,386	376,022
Equity Securities Available For Sale [Member] | Common Stock [Member]
Total Equity Securities Available For Sale	295,031	282,066
Equity Securities Available For Sale [Member] | Preferred Stock [Member]
Total Equity Securities Available For Sale	104,355	93,956
Arbitrage Trading Account [Member]
Fair Value, Assets Measured On Recurring Basis, Arbitrage Trading Account assets	701,223	329,077
Level 1 [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	530,463	514,261
Fair Value, Liabilities Measured On Recurring Basis, Securities Sold, Not Yet Purchased	70,580	114,909
Level 1 [Member] | Fixed Maturities Securities [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 1 [Member] | Fixed Maturities Securities [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 [Member] | State And Municipal [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 1 [Member] | Fixed Maturities Securities [Member] | Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 1 [Member] | Fixed Maturities Securities [Member] | Corporate [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 1 [Member] | Fixed Maturities Securities [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	293,793	280,658
Level 1 [Member] | Equity Securities Available For Sale [Member] | Common Stock [Member]
Total Equity Securities Available For Sale	293,793	280,658
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	236,670	233,603
Level 2 [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	12,074,470	11,969,821
Fair Value, Liabilities Measured On Recurring Basis, Securities Sold, Not Yet Purchased	9,383	6,558
Level 2 [Member] | Fixed Maturities Securities [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	11,507,045	11,781,940
Level 2 [Member] | Fixed Maturities Securities [Member] | U.S. Government And Government Agency [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	875,712	898,463
Level 2 [Member] | Fixed Maturities Securities [Member] | State And Municipal [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	4,409,616	4,768,519
Level 2 [Member] | Fixed Maturities Securities [Member] | Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	1,544,913	1,715,697
Level 2 [Member] | Fixed Maturities Securities [Member] | Corporate [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	3,515,310	3,287,654
Level 2 [Member] | Fixed Maturities Securities [Member] | Foreign [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	1,161,494	1,111,607
Level 2 [Member] | Equity Securities Available For Sale [Member]
Total Equity Securities Available For Sale	103,969	93,335
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	103,969	93,335
Level 2 [Member] | Arbitrage Trading Account [Member]
Fair Value, Assets Measured On Recurring Basis, Arbitrage Trading Account assets	463,456	94,546
Level 3 [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	58,490	62,022
Fair Value, Liabilities Measured On Recurring Basis, Securities Sold, Not Yet Purchased	2	20
Level 3 [Member] | Fixed Maturities Securities [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	55,769	59,065
Level 3 [Member] | Fixed Maturities Securities [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 [Member] | State And Municipal [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 3 [Member] | Fixed Maturities Securities [Member] | Mortgage-Backed Securities [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	0	0
Level 3 [Member] | Fixed Maturities Securities [Member] | Corporate [Member]
Fair Value Assets Measured On Recurring Basis Total Assets	55,769	59,065
Level 3 [Member] | Fixed Maturities Securities [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	1,624	2,029
Level 3 [Member] | Equity Securities Available For Sale [Member] | Common Stock [Member]
Total Equity Securities Available For Sale	1,238	1,408
Level 3 [Member] | Equity Securities Available For Sale [Member] | Preferred Stock [Member]
Total Equity Securities Available For Sale	386	621
Level 3 [Member] | Arbitrage Trading Account [Member]
Fair Value, Assets Measured On Recurring Basis, Arbitrage Trading Account assets	$ 1,097	$ 928