Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets:
Total fixed maturity securities available for sale	$ 13,794,294	$ 13,102,142
Equity securities available for sale	614,025	669,200
Arbitrage trading account	488,238	299,999
Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	13,794,294	13,102,142
Equity securities available for sale	614,025	669,200
Arbitrage trading account	488,238	299,999
Assets, fair value disclosure	14,896,557	14,071,341
Liabilities:
Securities sold but not yet purchased	44,937	51,179
Fair Value, Measurements, Recurring \| U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	411,605	513,802
Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	4,413,169	4,519,503
Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Total fixed maturity securities available for sale	1,320,119	1,189,645
Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	2,388,618	1,907,860
Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	4,320,374	4,068,527
Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	940,409	902,805
Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities available for sale	419,520	445,858
Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities available for sale	194,505	223,342
Level 1 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities available for sale	410,133	429,647
Arbitrage trading account	275,818	224,623
Assets, fair value disclosure	685,951	654,270
Liabilities:
Securities sold but not yet purchased	44,851	51,089
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 available for sale	410,133	429,647
Level 1 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities available for sale	0	0
Level 2 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	13,794,120	13,101,959
Equity securities available for sale	190,649	227,137
Arbitrage trading account	212,420	75,376
Assets, fair value disclosure	14,197,189	13,404,472
Liabilities:
Securities sold but not yet purchased	86	90
Level 2 \| Fair Value, Measurements, Recurring \| U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	411,605	513,802
Level 2 \| Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	4,413,169	4,519,503
Level 2 \| Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Total fixed maturity securities available for sale	1,320,119	1,189,645
Level 2 \| Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	2,388,444	1,907,677
Level 2 \| Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	4,320,374	4,068,527
Level 2 \| Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	940,409	902,805
Level 2 \| Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities available for sale	0	7,457
Level 2 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities available for sale	190,649	219,680
Level 3 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	174	183
Equity securities available for sale	13,243	12,416
Arbitrage trading account	0	0
Assets, fair value disclosure	13,417	12,599
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	174	183
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 available for sale	9,387	8,754
Level 3 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities available for sale	$ 3,856	$ 3,662

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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Amount of debt securities categorized neither as held-to-maturity nor trading.

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-Publisher FASB
-Name Accounting Standards Codification
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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 investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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