Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Assets:
Total fixed maturity securities available for sale	$ 23,575,885	$ 22,353,453
Equity securities	1,145,040	1,203,788
Arbitrage trading account	832,000	1,123,000
Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	23,575,885	22,353,453
Equity securities	1,145,040	1,203,788
Arbitrage trading account	831,705	1,122,599
Total	25,552,630	24,679,840
Liabilities:
Trading account securities sold but not yet purchased	52,407	73,358
Fair Value, Measurements, Recurring \| U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	2,926,978	2,235,341
Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	2,196,033	2,294,644
Fair Value, Measurements, Recurring \| Mortgage-backed
Assets:
Total fixed maturity securities available for sale	4,095,213	3,765,490
Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	3,971,671	3,885,012
Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	8,560,358	8,417,641
Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	1,825,632	1,755,325
Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	682,677	760,167
Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	462,363	443,621
Level 1 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	679,627	757,115
Arbitrage trading account	744,189	1,062,459
Total	1,423,816	1,819,574
Liabilities:
Trading account securities sold but not yet purchased	52,407	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	679,627	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	23,556,102	22,333,786
Equity securities	458,540	440,958
Arbitrage trading account	83,974	56,630
Total	24,098,616	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	2,926,978	2,235,341
Level 2 \| Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	2,196,033	2,294,644
Level 2 \| Fair Value, Measurements, Recurring \| Mortgage-backed
Assets:
Total fixed maturity securities available for sale	4,095,213	3,765,490
Level 2 \| Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	3,971,671	3,885,012
Level 2 \| Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	8,540,575	8,397,974
Level 2 \| Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	1,825,632	1,755,325
Level 2 \| Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	1,011	1,011
Level 2 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	457,529	439,947
Level 3 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	19,783	19,667
Equity securities	6,873	5,715
Arbitrage trading account	3,542	3,510
Total	30,198	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	19,783	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	2,039	2,041
Level 3 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	$ 4,834	$ 3,674

Fair value portion of asset recognized for present right to economic benefit.

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