Fair Value Measurements (Balances Of Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Thousands | Sep. 30, 2011 | Dec. 31, 2010 |
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | $ 5,576,584 | $ 5,431,226 |
| Total equity securities | 1,609,199 | 1,721,971 |
| Short-term investments | 491,756 | 325,340 |
| Total investments available-for-sale | 7,677,539 | 7,478,537 |
| Derivative contracts | 27,915 | 25,228 |
Residential Mortgage-Backed Securities [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 417,954 | 425,563 |
Residential Mortgage-Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
Residential Mortgage-Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 417,954 | 425,563 |
Residential Mortgage-Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
U.S. Treasury Securities And Obligations Of U.S. Government Agencies [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 308,159 | 321,338 |
U.S. Treasury Securities And Obligations Of U.S. Government Agencies [Member] | Fair Value, Inputs, Level 1 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
U.S. Treasury Securities And Obligations Of U.S. Government Agencies [Member] | Fair Value, Inputs, Level 2 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 308,159 | 321,338 |
U.S. Treasury Securities And Obligations Of U.S. Government Agencies [Member] | Fair Value, Inputs, Level 3 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
Obligations Of States, Municipalities And Political Subdivisions [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 2,918,163 | 2,799,339 |
Obligations Of States, Municipalities And Political Subdivisions [Member] | Fair Value, Inputs, Level 1 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
Obligations Of States, Municipalities And Political Subdivisions [Member] | Fair Value, Inputs, Level 2 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 2,918,163 | 2,799,339 |
Obligations Of States, Municipalities And Political Subdivisions [Member] | Fair Value, Inputs, Level 3 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
Asset-Backed Securities [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 17,552 | 22,756 |
Asset-Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
Asset-Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 17,552 | 22,756 |
Asset-Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
Convertible Bonds [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | | 16,725 |
Convertible Bonds [Member] | Fair Value, Inputs, Level 1 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | | 0 |
Convertible Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | | 16,725 |
Convertible Bonds [Member] | Fair Value, Inputs, Level 3 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | | 0 |
Public Utilities [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 76,107 | 102,444 |
Public Utilities [Member] | Fair Value, Inputs, Level 1 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
Public Utilities [Member] | Fair Value, Inputs, Level 2 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 76,107 | 102,444 |
Public Utilities [Member] | Fair Value, Inputs, Level 3 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
Insurance Companies, Banks And Trusts [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total equity securities | 612,395 | 710,986 |
Insurance Companies, Banks And Trusts [Member] | Fair Value, Inputs, Level 1 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total equity securities | 612,395 | 710,986 |
Insurance Companies, Banks And Trusts [Member] | Fair Value, Inputs, Level 2 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total equity securities | 0 | 0 |
Insurance Companies, Banks And Trusts [Member] | Fair Value, Inputs, Level 3 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total equity securities | 0 | 0 |
Industrial, Consumer And All Other [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total equity securities | 996,804 | 1,010,985 |
Industrial, Consumer And All Other [Member] | Fair Value, Inputs, Level 1 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total equity securities | 996,804 | 1,010,985 |
Industrial, Consumer And All Other [Member] | Fair Value, Inputs, Level 2 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total equity securities | 0 | 0 |
Industrial, Consumer And All Other [Member] | Fair Value, Inputs, Level 3 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total equity securities | 0 | 0 |
Foreign Governments [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 740,363 | 572,818 |
Foreign Governments [Member] | Fair Value, Inputs, Level 1 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
Foreign Governments [Member] | Fair Value, Inputs, Level 2 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 740,363 | 572,818 |
Foreign Governments [Member] | Fair Value, Inputs, Level 3 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
All Other Corporate Bonds [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 1,098,286 | 1,170,243 |
All Other Corporate Bonds [Member] | Fair Value, Inputs, Level 1 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
All Other Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 1,098,286 | 1,170,243 |
All Other Corporate Bonds [Member] | Fair Value, Inputs, Level 3 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
Fair Value, Inputs, Level 1 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
| Total equity securities | 1,609,199 | 1,721,971 |
| Short-term investments | 428,681 | 269,466 |
| Total investments available-for-sale | 2,037,880 | 1,991,437 |
| Derivative contracts | 0 | 0 |
Fair Value, Inputs, Level 2 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 5,576,584 | 5,431,226 |
| Total equity securities | 0 | 0 |
| Short-term investments | 63,075 | 55,874 |
| Total investments available-for-sale | 5,639,659 | 5,487,100 |
| Derivative contracts | 0 | 0 |
Fair Value, Inputs, Level 3 [Member] | | |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
| Total fixed maturities | 0 | 0 |
| Total equity securities | 0 | 0 |
| Short-term investments | 0 | 0 |
| Total investments available-for-sale | 0 | 0 |
| Derivative contracts | $ 27,915 | $ 25,228 |