Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets:
Total fixed maturity securities available for sale	$ 15,999,361	$ 14,086,595
Equity securities	818,738	625,667
Arbitrage trading account	860,000	341,000
Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	15,999,361	14,086,595
Equity securities	818,738	625,667
Arbitrage trading account	860,339	341,473
Assets, fair value disclosure	17,678,438	15,053,735
Liabilities:
Trading account securities sold but not yet purchased	739	10,048
Fair Value, Measurements, Recurring \| U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	518,333	603,871
Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	3,329,345	3,620,664
Fair Value, Measurements, Recurring \| Mortgage-backed
Assets:
Total fixed maturity securities available for sale	973,337	1,020,330
Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	4,655,555	3,194,586
Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	5,424,064	4,671,581
Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	1,098,727	975,563
Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	609,939	350,181
Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	208,799	275,486
Level 1 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	600,672	340,966
Arbitrage trading account	837,237	298,359
Assets, fair value disclosure	1,437,909	639,325
Liabilities:
Trading account securities sold but not yet purchased	739	10,048
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	600,672	340,966
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	15,999,361	14,085,595
Equity securities	199,466	266,155
Arbitrage trading account	23,102	43,114
Assets, fair value disclosure	16,221,929	14,394,864
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	518,333	603,871
Level 2 \| Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	3,329,345	3,620,664
Level 2 \| Fair Value, Measurements, Recurring \| Mortgage-backed
Assets:
Total fixed maturity securities available for sale	973,337	1,020,330
Level 2 \| Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	4,655,555	3,194,586
Level 2 \| Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	5,424,064	4,670,581
Level 2 \| Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	1,098,727	975,563
Level 2 \| Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	0	0
Level 2 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	199,466	266,155
Level 3 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	1,000
Equity securities	18,600	18,546
Arbitrage trading account	0	0
Assets, fair value disclosure	18,600	19,546
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	1,000
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	9,267	9,215
Level 3 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	$ 9,333	$ 9,331

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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-Name Accounting Standards Codification
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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).

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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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