Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Total fixed maturity securities available for sale	$ 13,820,529	$ 14,102,278
Equity securities	435,303	480,620
Arbitrage trading account	595,727	400,809
Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	13,820,529	14,102,278
Equity securities	435,303	480,620
Arbitrage trading account	595,727	400,809
Assets, fair value disclosure	14,851,559	14,983,707
Liabilities:
Trading account securities sold but not yet purchased	19,254	36,143
Fair Value, Measurements, Recurring | U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	697,432	786,931
Fair Value, Measurements, Recurring | State and municipal
Assets:
Total fixed maturity securities available for sale	3,625,395	3,895,632
Fair Value, Measurements, Recurring | Mortgage-backed securities
Assets:
Total fixed maturity securities available for sale	1,082,379	1,625,594
Fair Value, Measurements, Recurring | Asset-backed
Assets:
Total fixed maturity securities available for sale	3,306,439	2,790,630
Fair Value, Measurements, Recurring | Corporate
Assets:
Total fixed maturity securities available for sale	4,239,540	4,156,415
Fair Value, Measurements, Recurring | Foreign government
Assets:
Total fixed maturity securities available for sale	869,344	847,076
Fair Value, Measurements, Recurring | Common stocks
Assets:
Equity securities	160,858	166,805
Fair Value, Measurements, Recurring | Preferred stocks
Assets:
Equity securities	274,445	313,815
Level 1 | Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	152,508	157,752
Arbitrage trading account	307,915	381,061
Assets, fair value disclosure	460,423	538,813
Liabilities:
Trading account securities sold but not yet purchased	19,254	36,143
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 securities
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	152,508	157,752
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	13,820,529	14,102,278
Equity securities	265,117	307,310
Arbitrage trading account	287,812	19,748
Assets, fair value disclosure	14,373,458	14,429,336
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	697,432	786,931
Level 2 | Fair Value, Measurements, Recurring | State and municipal
Assets:
Total fixed maturity securities available for sale	3,625,395	3,895,632
Level 2 | Fair Value, Measurements, Recurring | Mortgage-backed securities
Assets:
Total fixed maturity securities available for sale	1,082,379	1,625,594
Level 2 | Fair Value, Measurements, Recurring | Asset-backed
Assets:
Total fixed maturity securities available for sale	3,306,439	2,790,630
Level 2 | Fair Value, Measurements, Recurring | Corporate
Assets:
Total fixed maturity securities available for sale	4,239,540	4,156,415
Level 2 | Fair Value, Measurements, Recurring | Foreign government
Assets:
Total fixed maturity securities available for sale	869,344	847,076
Level 2 | Fair Value, Measurements, Recurring | Common stocks
Assets:
Equity securities	0	0
Level 2 | Fair Value, Measurements, Recurring | Preferred stocks
Assets:
Equity securities	265,117	307,310
Level 3 | Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	17,678	15,558
Arbitrage trading account	0	0
Assets, fair value disclosure	17,678	15,558
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 securities
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	8,350	9,053
Level 3 | Fair Value, Measurements, Recurring | Preferred stocks
Assets:
Equity securities	$ 9,328	$ 6,505