Investments (Available-For-Sale Investments) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost	$ 5,125,773	$ 5,172,952
Equity securities, Amortized Cost	1,252,214	1,156,294
Available-for-sale investments, Amortized Cost	6,841,684	6,870,260
Available-for-sale investments, Gross Unrealized Holding Gains	1,307,964	1,100,525
Available-for-sale investments, Gross Unrealized Holding Losses	(2,393)	(8,798)
Available-for-sale investments, Unrealized Other-Than-Temporary Impairment Losses	(9,056)	(8,872)
Total fixed maturities, Estimated Fair Value	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
U.S. Treasury securities and obligations Of U.S. government agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost	325,998	299,413
Available-for-sale investments, Gross Unrealized Holding Gains	20,302	22,789
Available-for-sale investments, Gross Unrealized Holding Losses	(222)	(9)
Available-for-sale investments, Unrealized Other-Than-Temporary Impairment Losses	0	0
Total fixed maturities, Estimated Fair Value	346,078	322,193
Obligations of states, municipalities and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost	2,709,251	2,729,838
Available-for-sale investments, Gross Unrealized Holding Gains	208,549	201,477
Available-for-sale investments, Gross Unrealized Holding Losses	(924)	(794)
Available-for-sale investments, Unrealized Other-Than-Temporary Impairment Losses	0	0
Total fixed maturities, Estimated Fair Value	2,916,876	2,930,521
Foreign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost	576,187	572,253
Available-for-sale investments, Gross Unrealized Holding Gains	44,863	45,629
Available-for-sale investments, Gross Unrealized Holding Losses	(331)	(1,068)
Available-for-sale investments, Unrealized Other-Than-Temporary Impairment Losses	0	0
Total fixed maturities, Estimated Fair Value	620,719	616,814
Residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost	331,526	366,859
Available-for-sale investments, Gross Unrealized Holding Gains	23,990	24,601
Available-for-sale investments, Gross Unrealized Holding Losses	(5)	(18)
Available-for-sale investments, Unrealized Other-Than-Temporary Impairment Losses	(2,258)	(2,258)
Total fixed maturities, Estimated Fair Value	353,253	389,184
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost	15,779	16,096
Available-for-sale investments, Gross Unrealized Holding Gains	688	731
Available-for-sale investments, Gross Unrealized Holding Losses	0	(9)
Available-for-sale investments, Unrealized Other-Than-Temporary Impairment Losses	0	0
Total fixed maturities, Estimated Fair Value	16,467	16,818
Public utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost	65,294	63,965
Available-for-sale investments, Gross Unrealized Holding Gains	5,536	5,462
Available-for-sale investments, Gross Unrealized Holding Losses	0	0
Available-for-sale investments, Unrealized Other-Than-Temporary Impairment Losses	0	0
Total fixed maturities, Estimated Fair Value	70,830	69,427
All other corporate bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost	1,101,738	1,124,528
Available-for-sale investments, Gross Unrealized Holding Gains	88,804	78,053
Available-for-sale investments, Gross Unrealized Holding Losses	(303)	(2,750)
Available-for-sale investments, Unrealized Other-Than-Temporary Impairment Losses	(6,798)	(6,614)
Total fixed maturities, Estimated Fair Value	1,183,441	1,193,217
Total fixed maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost	5,125,773	5,172,952
Available-for-sale investments, Gross Unrealized Holding Gains	392,732	378,742
Available-for-sale investments, Gross Unrealized Holding Losses	(1,785)	(4,648)
Available-for-sale investments, Unrealized Other-Than-Temporary Impairment Losses	(9,056)	(8,872)
Total fixed maturities, Estimated Fair Value	5,507,664	5,538,174
Insurance companies, banks and trusts [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity securities, Amortized Cost	417,044	389,421
Available-for-sale investments, Gross Unrealized Holding Gains	351,298	296,648
Available-for-sale investments, Gross Unrealized Holding Losses	0	(1,366)
Available-for-sale investments, Unrealized Other-Than-Temporary Impairment Losses	0	0
Total equity securities	768,342	684,703
Industrial, consumer and all other [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity securities, Amortized Cost	835,170	766,873
Available-for-sale investments, Gross Unrealized Holding Gains	563,930	425,131
Available-for-sale investments, Gross Unrealized Holding Losses	(601)	(2,780)
Available-for-sale investments, Unrealized Other-Than-Temporary Impairment Losses	0	0
Total equity securities	1,398,499	1,189,224
Total equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity securities, Amortized Cost	1,252,214	1,156,294
Available-for-sale investments, Gross Unrealized Holding Gains	915,228	721,779
Available-for-sale investments, Gross Unrealized Holding Losses	(601)	(4,146)
Available-for-sale investments, Unrealized Other-Than-Temporary Impairment Losses	0	0
Total equity securities	2,166,841	1,873,927
Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Short-term investments, Amortized Cost	463,697	541,014
Available-for-sale investments, Gross Unrealized Holding Gains	4	4
Available-for-sale investments, Gross Unrealized Holding Losses	(7)	(4)
Available-for-sale investments, Unrealized Other-Than-Temporary Impairment Losses	0	0
Short-term investments	$ 463,694	$ 541,014