Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Assets:
Total fixed maturity securities available for sale	$ 16,351,954	$ 16,528,692
Equity securities	1,126,491	941,243
Arbitrage trading account	1,189,000	1,180,000
Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	16,351,954	16,528,692
Equity securities	1,126,491	941,243
Arbitrage trading account	1,188,910	1,179,606
Assets, fair value disclosure	18,667,355	18,649,541
Liabilities:
Trading account securities sold but not yet purchased	241	1,169
Fair Value, Measurements, Recurring \| U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	818,848	855,343
Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	3,055,874	3,304,133
Fair Value, Measurements, Recurring \| Mortgage-backed
Assets:
Total fixed maturity securities available for sale	1,384,736	1,068,075
Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	4,257,421	4,490,565
Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	5,588,507	5,595,675
Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	1,246,568	1,214,901
Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	883,317	695,403
Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	243,174	245,840
Level 1 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	877,387	684,470
Arbitrage trading account	1,186,353	1,153,079
Assets, fair value disclosure	2,063,740	1,837,549
Liabilities:
Trading account securities sold but not yet purchased	241	1,137
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	877,387	684,470
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	16,351,954	16,528,692
Equity securities	232,433	236,183
Arbitrage trading account	2,557	26,527
Assets, fair value disclosure	16,586,944	16,791,402
Liabilities:
Trading account securities sold but not yet purchased	0	32
Level 2 \| Fair Value, Measurements, Recurring \| U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	818,848	855,343
Level 2 \| Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	3,055,874	3,304,133
Level 2 \| Fair Value, Measurements, Recurring \| Mortgage-backed
Assets:
Total fixed maturity securities available for sale	1,384,736	1,068,075
Level 2 \| Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	4,257,421	4,490,565
Level 2 \| Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	5,588,507	5,595,675
Level 2 \| Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	1,246,568	1,214,901
Level 2 \| Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	1,230	1,639
Level 2 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	231,203	234,544
Level 3 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	16,671	20,590
Arbitrage trading account	0	0
Assets, fair value disclosure	16,671	20,590
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	4,700	9,294
Level 3 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	$ 11,971	$ 11,296

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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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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