Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Assets:
Total fixed maturity securities available for sale	$ 20,910,872	$ 20,124,936
Equity securities	1,178,048	1,090,347
Arbitrage trading account	1,146,000	938,000
Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	20,910,872	20,124,936
Equity securities	1,178,048	1,090,347
Arbitrage trading account	1,146,119	938,049
Total	23,235,039	22,153,332
Liabilities:
Trading account securities sold but not yet purchased	40,298	9,357
Fair Value, Measurements, Recurring \| U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	1,829,552	1,716,731
Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	2,552,786	2,634,422
Fair Value, Measurements, Recurring \| Mortgage-backed
Assets:
Total fixed maturity securities available for sale	2,408,018	2,266,455
Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	4,239,561	4,187,040
Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	8,162,703	7,654,059
Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	1,718,252	1,666,229
Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	846,669	838,054
Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	331,379	252,293
Level 1 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	843,441	835,338
Arbitrage trading account	752,159	546,110
Total	1,595,600	1,381,448
Liabilities:
Trading account securities sold but not yet purchased	40,298	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	843,441	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	20,910,872	20,124,936
Equity securities	328,802	249,756
Arbitrage trading account	390,284	388,167
Total	21,629,958	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	1,829,552	1,716,731
Level 2 \| Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	2,552,786	2,634,422
Level 2 \| Fair Value, Measurements, Recurring \| Mortgage-backed
Assets:
Total fixed maturity securities available for sale	2,408,018	2,266,455
Level 2 \| Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	4,239,561	4,187,040
Level 2 \| Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	8,162,703	7,654,059
Level 2 \| Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	1,718,252	1,666,229
Level 2 \| Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	1,059	1,158
Level 2 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	327,743	248,598
Level 3 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	5,805	5,253
Arbitrage trading account	3,676	3,772
Total	9,481	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	2,169	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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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), classified as current.

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