Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets:
Total fixed maturity securities available for sale	$ 13,587,315	$ 14,102,278
Equity securities	356,102	480,620
Arbitrage trading account	666,829	400,809
Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	13,587,315	14,102,278
Equity securities	356,102	480,620
Arbitrage trading account	666,829	400,809
Assets, fair value disclosure	14,610,246	14,983,707
Liabilities:
Securities sold but not yet purchased	14,003	36,143
Fair Value, Measurements, Recurring \| U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	739,662	786,931
Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	3,748,155	3,895,632
Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Total fixed maturity securities available for sale	1,455,440	1,625,594
Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	2,938,052	2,790,630
Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	3,978,517	4,156,415
Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	727,489	847,076
Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	146,658	166,805
Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	209,444	313,815
Level 1 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	137,791	157,752
Arbitrage trading account	480,495	381,061
Assets, fair value disclosure	618,286	538,813
Liabilities:
Securities sold but not yet purchased	14,003	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	137,791	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	13,587,315	14,102,278
Equity securities	202,939	307,310
Arbitrage trading account	186,334	19,748
Assets, fair value disclosure	13,976,588	14,429,336
Liabilities:
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	739,662	786,931
Level 2 \| Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	3,748,155	3,895,632
Level 2 \| Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Total fixed maturity securities available for sale	1,455,440	1,625,594
Level 2 \| Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	2,938,052	2,790,630
Level 2 \| Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	3,978,517	4,156,415
Level 2 \| Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	727,489	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	202,939	307,310
Level 3 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	15,372	15,558
Arbitrage trading account	0	0
Assets, fair value disclosure	15,372	15,558
Liabilities:
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	8,867	9,053
Level 3 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	$ 6,505	$ 6,505

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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-Publisher FASB
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Amount of equity securities categorized neither as held-to-maturity nor as trading.

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Fair value portion of liabilities pertaining to principal and customer trading transactions, or which may be incurred with the objective of generating a profit from short-term fluctuations in price as part of an entity's market-making, hedging and proprietary trading. Examples include, but are not limited to, short positions in securities, derivatives and commodities.

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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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