Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Assets:
Total fixed maturity securities available for sale	$ 24,063,105	$ 22,353,453
Equity securities	1,262,192	1,203,788
Arbitrage trading account	1,035,000	1,123,000
Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	24,063,105	22,353,453
Equity securities	1,262,192	1,203,788
Arbitrage trading account	1,034,557	1,122,599
Total	26,359,854	24,679,840
Liabilities:
Trading account securities sold but not yet purchased	30,909	73,358
Fair Value, Measurements, Recurring | U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	3,101,062	2,235,341
Fair Value, Measurements, Recurring | State and municipal
Assets:
Total fixed maturity securities available for sale	2,146,601	2,294,644
Fair Value, Measurements, Recurring | Mortgage-backed
Assets:
Total fixed maturity securities available for sale	4,327,577	3,765,490
Fair Value, Measurements, Recurring | Asset-backed
Assets:
Total fixed maturity securities available for sale	3,853,435	3,885,012
Fair Value, Measurements, Recurring | Corporate
Assets:
Total fixed maturity securities available for sale	8,758,776	8,417,641
Fair Value, Measurements, Recurring | Foreign government
Assets:
Total fixed maturity securities available for sale	1,875,654	1,755,325
Fair Value, Measurements, Recurring | Common stocks
Assets:
Equity securities	719,570	760,167
Fair Value, Measurements, Recurring | Preferred stocks
Assets:
Equity securities	542,622	443,621
Level 1 | Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	716,561	757,115
Arbitrage trading account	905,421	1,062,459
Total	1,621,982	1,819,574
Liabilities:
Trading account securities sold but not yet purchased	30,909	73,358
Level 1 | Fair Value, Measurements, Recurring | U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	0	0
Level 1 | Fair Value, Measurements, Recurring | State and municipal
Assets:
Total fixed maturity securities available for sale	0	0
Level 1 | Fair Value, Measurements, Recurring | Mortgage-backed
Assets:
Total fixed maturity securities available for sale	0	0
Level 1 | Fair Value, Measurements, Recurring | Asset-backed
Assets:
Total fixed maturity securities available for sale	0	0
Level 1 | Fair Value, Measurements, Recurring | Corporate
Assets:
Total fixed maturity securities available for sale	0	0
Level 1 | Fair Value, Measurements, Recurring | Foreign government
Assets:
Total fixed maturity securities available for sale	0	0
Level 1 | Fair Value, Measurements, Recurring | Common stocks
Assets:
Equity securities	716,561	757,115
Level 1 | Fair Value, Measurements, Recurring | Preferred stocks
Assets:
Equity securities	0	0
Level 2 | Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	24,043,129	22,333,786
Equity securities	534,806	440,958
Arbitrage trading account	125,391	56,630
Total	24,703,326	22,831,374
Liabilities:
Trading account securities sold but not yet purchased	0	0
Level 2 | Fair Value, Measurements, Recurring | U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	3,101,062	2,235,341
Level 2 | Fair Value, Measurements, Recurring | State and municipal
Assets:
Total fixed maturity securities available for sale	2,146,601	2,294,644
Level 2 | Fair Value, Measurements, Recurring | Mortgage-backed
Assets:
Total fixed maturity securities available for sale	4,327,577	3,765,490
Level 2 | Fair Value, Measurements, Recurring | Asset-backed
Assets:
Total fixed maturity securities available for sale	3,853,435	3,885,012
Level 2 | Fair Value, Measurements, Recurring | Corporate
Assets:
Total fixed maturity securities available for sale	8,738,800	8,397,974
Level 2 | Fair Value, Measurements, Recurring | Foreign government
Assets:
Total fixed maturity securities available for sale	1,875,654	1,755,325
Level 2 | Fair Value, Measurements, Recurring | Common stocks
Assets:
Equity securities	858	1,011
Level 2 | Fair Value, Measurements, Recurring | Preferred stocks
Assets:
Equity securities	533,948	439,947
Level 3 | Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	19,976	19,667
Equity securities	10,825	5,715
Arbitrage trading account	3,745	3,510
Total	34,546	28,892
Liabilities:
Trading account securities sold but not yet purchased	0	0
Level 3 | Fair Value, Measurements, Recurring | U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	0	0
Level 3 | Fair Value, Measurements, Recurring | State and municipal
Assets:
Total fixed maturity securities available for sale	0	0
Level 3 | Fair Value, Measurements, Recurring | Mortgage-backed
Assets:
Total fixed maturity securities available for sale	0	0
Level 3 | Fair Value, Measurements, Recurring | Asset-backed
Assets:
Total fixed maturity securities available for sale	0	0
Level 3 | Fair Value, Measurements, Recurring | Corporate
Assets:
Total fixed maturity securities available for sale	19,976	19,667
Level 3 | Fair Value, Measurements, Recurring | Foreign government
Assets:
Total fixed maturity securities available for sale	0	0
Level 3 | Fair Value, Measurements, Recurring | Common stocks
Assets:
Equity securities	2,151	2,041
Level 3 | Fair Value, Measurements, Recurring | Preferred stocks
Assets:
Equity securities	$ 8,674	$ 3,674