Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Assets:
Total fixed maturity securities available for sale	$ 18,039,347	$ 17,536,053
Equity securities	1,279,955	1,185,894
Equity securities	609,000	944,000
Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	18,039,347	17,536,053
Equity securities	1,279,955	1,185,894
Equity securities	609,001	944,230
Assets, fair value disclosure	19,928,303	19,666,177
Fair Value, Measurements, Recurring | U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	1,049,267	892,258
Fair Value, Measurements, Recurring | State and municipal
Assets:
Total fixed maturity securities available for sale	2,789,359	2,890,098
Fair Value, Measurements, Recurring | Mortgage-backed
Assets:
Total fixed maturity securities available for sale	1,897,729	1,665,410
Fair Value, Measurements, Recurring | Asset-backed
Assets:
Total fixed maturity securities available for sale	3,845,453	3,982,773
Fair Value, Measurements, Recurring | Corporate
Assets:
Total fixed maturity securities available for sale	7,125,649	6,703,992
Fair Value, Measurements, Recurring | Foreign government
Assets:
Total fixed maturity securities available for sale	1,331,890	1,401,522
Fair Value, Measurements, Recurring | Common stocks
Assets:
Equity securities	1,057,599	982,751
Fair Value, Measurements, Recurring | Preferred stocks
Assets:
Equity securities	222,356	203,143
Level 1 | Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	1,054,268	978,991
Equity securities	574,096	822,192
Assets, fair value disclosure	1,628,364	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	1,054,268	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	18,039,347	17,536,053
Equity securities	212,219	193,005
Equity securities	31,251	118,448
Assets, fair value disclosure	18,282,817	17,847,506
Level 2 | Fair Value, Measurements, Recurring | U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	1,049,267	892,258
Level 2 | Fair Value, Measurements, Recurring | State and municipal
Assets:
Total fixed maturity securities available for sale	2,789,359	2,890,098
Level 2 | Fair Value, Measurements, Recurring | Mortgage-backed
Assets:
Total fixed maturity securities available for sale	1,897,729	1,665,410
Level 2 | Fair Value, Measurements, Recurring | Asset-backed
Assets:
Total fixed maturity securities available for sale	3,845,453	3,982,773
Level 2 | Fair Value, Measurements, Recurring | Corporate
Assets:
Total fixed maturity securities available for sale	7,125,649	6,703,992
Level 2 | Fair Value, Measurements, Recurring | Foreign government
Assets:
Total fixed maturity securities available for sale	1,331,890	1,401,522
Level 2 | Fair Value, Measurements, Recurring | Common stocks
Assets:
Equity securities	1,162	1,161
Level 2 | Fair Value, Measurements, Recurring | Preferred stocks
Assets:
Equity securities	211,057	191,844
Level 3 | Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	13,468	13,898
Equity securities	3,654	3,590
Assets, fair value disclosure	17,122	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	2,169	2,599
Level 3 | Fair Value, Measurements, Recurring | Preferred stocks
Assets:
Equity securities	$ 11,299	$ 11,299