Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Assets:
Total fixed maturity securities available for sale	$ 22,619,949	$ 20,124,936
Equity securities	1,048,126	1,090,347
Arbitrage trading account	821,000	938,000
Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	22,619,949	20,124,936
Equity securities	1,048,126	1,090,347
Arbitrage trading account	820,928	938,049
Total	24,489,003	22,153,332
Liabilities:
Trading account securities sold but not yet purchased	36,093	9,357
Fair Value, Measurements, Recurring \| U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	2,070,985	1,716,731
Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	2,516,302	2,634,422
Fair Value, Measurements, Recurring \| Mortgage-backed
Assets:
Total fixed maturity securities available for sale	3,237,972	2,266,455
Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	4,107,321	4,187,040
Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	8,852,764	7,654,059
Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	1,834,605	1,666,229
Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	701,667	838,054
Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	346,459	252,293
Level 1 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	698,364	835,338
Arbitrage trading account	728,783	546,110
Total	1,427,147	1,381,448
Liabilities:
Trading account securities sold but not yet purchased	36,093	9,357
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	698,364	835,338
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	22,619,949	20,124,936
Equity securities	343,953	249,756
Arbitrage trading account	88,397	388,167
Total	23,052,299	20,762,859
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	2,070,985	1,716,731
Level 2 \| Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	2,516,302	2,634,422
Level 2 \| Fair Value, Measurements, Recurring \| Mortgage-backed
Assets:
Total fixed maturity securities available for sale	3,237,972	2,266,455
Level 2 \| Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	4,107,321	4,187,040
Level 2 \| Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	8,852,764	7,654,059
Level 2 \| Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	1,834,605	1,666,229
Level 2 \| Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	1,130	1,158
Level 2 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	342,823	248,598
Level 3 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	5,809	5,253
Arbitrage trading account	3,748	3,772
Total	9,557	9,025
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	0	0
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	2,173	1,558
Level 3 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	$ 3,636	$ 3,695

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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