Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Assets:
Total fixed maturity securities available for sale	$ 24,917,629	$ 22,353,453
Equity securities	1,382,432	1,203,788
Arbitrage trading account	1,070,000	1,123,000
Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	24,917,629	22,353,453
Equity securities	1,382,432	1,203,788
Arbitrage trading account	1,070,304	1,122,599
Total	27,370,365	24,679,840
Liabilities:
Trading account securities sold but not yet purchased	34,509	73,358
Fair Value, Measurements, Recurring \| U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	3,478,237	2,235,341
Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	1,974,136	2,294,644
Fair Value, Measurements, Recurring \| Mortgage-backed
Assets:
Total fixed maturity securities available for sale	4,969,446	3,765,490
Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	3,831,174	3,885,012
Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	8,740,772	8,417,641
Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	1,923,864	1,755,325
Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	721,950	760,167
Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	660,482	443,621
Level 1 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	718,999	757,115
Arbitrage trading account	881,153	1,062,459
Total	1,600,152	1,819,574
Liabilities:
Trading account securities sold but not yet purchased	34,509	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	718,999	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	24,897,610	22,333,786
Equity securities	652,650	440,958
Arbitrage trading account	189,038	56,630
Total	25,739,298	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	3,478,237	2,235,341
Level 2 \| Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	1,974,136	2,294,644
Level 2 \| Fair Value, Measurements, Recurring \| Mortgage-backed
Assets:
Total fixed maturity securities available for sale	4,969,446	3,765,490
Level 2 \| Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	3,831,174	3,885,012
Level 2 \| Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	8,720,753	8,397,974
Level 2 \| Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	1,923,864	1,755,325
Level 2 \| Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	842	1,011
Level 2 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	651,808	439,947
Level 3 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	20,019	19,667
Equity securities	10,783	5,715
Arbitrage trading account	113	3,510
Total	30,915	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	20,019	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	2,109	2,041
Level 3 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	$ 8,674	$ 3,674

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

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