Fair Value Measurements (Balances Of Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	$ 5,507,664	$ 5,538,174
Total equity securities	2,166,841	1,873,927
Short-term investments	463,694	541,014
Available-for-sale investments, Estimated Fair Value	8,138,199	7,953,115
Derivative contracts	18,300	29,300
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	346,078	322,193
Obligations of states, municipalities and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	2,916,876	2,930,521
Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	620,719	616,814
Residential mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	353,253	389,184
Asset-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	16,467	16,818
Public utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	70,830	69,427
All other corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	1,183,441	1,193,217
Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	2,166,841	1,873,927
Insurance companies, banks and trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	768,342	684,703
Industrial, consumer and all other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	1,398,499	1,189,224
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	5,507,664	5,538,174
Total equity securities	2,166,841	1,873,927
Short-term investments	463,694	541,014
Available-for-sale investments, Estimated Fair Value	8,138,199	7,953,115
Derivative contracts	18,270	29,331
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Total equity securities	2,166,841	1,873,927
Short-term investments	401,162	477,348
Available-for-sale investments, Estimated Fair Value	2,568,003	2,351,275
Derivative contracts	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	5,507,664	5,538,174
Total equity securities	0	0
Short-term investments	62,532	63,666
Available-for-sale investments, Estimated Fair Value	5,570,196	5,601,840
Derivative contracts	0	0
Fair Value, Measurements, Recurring [Member] | 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
Available-for-sale investments, Estimated Fair Value	0	0
Derivative contracts	18,270	29,331
Fair Value, Measurements, Recurring [Member] | 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	346,078	322,193
Fair Value, Measurements, Recurring [Member] | U.S. Treasury securities and obligations Of U.S. government agencies [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | U.S. Treasury securities and obligations Of U.S. government agencies [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	346,078	322,193
Fair Value, Measurements, Recurring [Member] | U.S. Treasury securities and obligations Of U.S. government agencies [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Obligations of states, municipalities and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	2,916,876	2,930,521
Fair Value, Measurements, Recurring [Member] | Obligations of states, municipalities and political subdivisions [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Obligations of states, municipalities and political subdivisions [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	2,916,876	2,930,521
Fair Value, Measurements, Recurring [Member] | Obligations of states, municipalities and political subdivisions [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	620,719	616,814
Fair Value, Measurements, Recurring [Member] | Foreign governments [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Foreign governments [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	620,719	616,814
Fair Value, Measurements, Recurring [Member] | Foreign governments [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	353,253	389,184
Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	353,253	389,184
Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Asset-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	16,467	16,818
Fair Value, Measurements, Recurring [Member] | Asset-backed securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Asset-backed securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	16,467	16,818
Fair Value, Measurements, Recurring [Member] | Asset-backed securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Public utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	70,830	69,427
Fair Value, Measurements, Recurring [Member] | Public utilities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Public utilities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	70,830	69,427
Fair Value, Measurements, Recurring [Member] | Public utilities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | All other corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	1,183,441	1,193,217
Fair Value, Measurements, Recurring [Member] | All other corporate bonds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | All other corporate bonds [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	1,183,441	1,193,217
Fair Value, Measurements, Recurring [Member] | All other corporate bonds [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total fixed maturities	0	0
Fair Value, Measurements, Recurring [Member] | Insurance companies, banks and trusts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	768,342	684,703
Fair Value, Measurements, Recurring [Member] | Insurance companies, banks and trusts [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	768,342	684,703
Fair Value, Measurements, Recurring [Member] | Insurance companies, banks and trusts [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	0	0
Fair Value, Measurements, Recurring [Member] | Insurance companies, banks and trusts [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	0	0
Fair Value, Measurements, Recurring [Member] | Industrial, consumer and all other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	1,398,499	1,189,224
Fair Value, Measurements, Recurring [Member] | Industrial, consumer and all other [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	1,398,499	1,189,224
Fair Value, Measurements, Recurring [Member] | Industrial, consumer and all other [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	0	0
Fair Value, Measurements, Recurring [Member] | Industrial, consumer and all other [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	$ 0	$ 0