Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Assets:
Total fixed maturity securities available for sale	$ 18,003,312	$ 17,536,053
Equity securities	1,239,712	1,185,894
Arbitrage trading account	724,000	944,000
Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	18,003,312	17,536,053
Equity securities	1,239,712	1,185,894
Arbitrage trading account	723,967	944,230
Total	19,966,991	19,666,177
Fair Value, Measurements, Recurring \| U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	1,238,117	892,258
Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	2,703,044	2,890,098
Fair Value, Measurements, Recurring \| Mortgage-backed
Assets:
Total fixed maturity securities available for sale	1,903,672	1,665,410
Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	3,743,803	3,982,773
Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	7,007,068	6,703,992
Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	1,407,608	1,401,522
Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	1,014,820	982,751
Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	224,892	203,143
Level 1 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	1,011,449	978,991
Arbitrage trading account	610,553	822,192
Total	1,622,002	1,801,183
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	1,011,449	978,991
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	18,003,312	17,536,053
Equity securities	214,746	193,005
Arbitrage trading account	109,749	118,448
Total	18,327,807	17,847,506
Level 2 \| Fair Value, Measurements, Recurring \| U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	1,238,117	892,258
Level 2 \| Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	2,703,044	2,890,098
Level 2 \| Fair Value, Measurements, Recurring \| Mortgage-backed
Assets:
Total fixed maturity securities available for sale	1,903,672	1,665,410
Level 2 \| Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	3,743,803	3,982,773
Level 2 \| Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	7,007,068	6,703,992
Level 2 \| Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	1,407,608	1,401,522
Level 2 \| Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	1,153	1,161
Level 2 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	213,593	191,844
Level 3 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	13,517	13,898
Arbitrage trading account	3,665	3,590
Total	17,182	17,488
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	2,218	2,599
Level 3 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	$ 11,299	$ 11,299

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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