Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets:
Total fixed maturity securities available for sale	$ 13,139,643	$ 13,471,918
Equity securities	472,389	576,647
Arbitrage trading account	721,088	617,649
Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	13,139,643	13,471,918
Equity securities	472,389	576,647
Arbitrage trading account	721,088	617,649
Assets, fair value disclosure	14,333,120	14,666,214
Liabilities:
Securities sold but not yet purchased	90,281	64,358
Fair Value, Measurements, Recurring \| U.S. government and government agency
Assets:
Total fixed maturity securities available for sale	481,633	377,740
Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	4,064,854	4,445,094
Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Total fixed maturity securities available for sale	1,458,512	1,299,931
Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	2,354,915	2,111,544
Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	3,978,189	4,389,112
Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	801,540	848,497
Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	288,027	352,204
Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	184,362	224,443
Level 1 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	0	0
Equity securities	279,102	342,834
Arbitrage trading account	446,901	471,420
Assets, fair value disclosure	726,003	814,254
Liabilities:
Securities sold but not yet purchased	90,281	64,358
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	279,102	342,834
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	13,139,542	13,471,746
Equity securities	173,518	213,600
Arbitrage trading account	274,187	146,229
Assets, fair value disclosure	13,587,247	13,831,575
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	481,633	377,740
Level 2 \| Fair Value, Measurements, Recurring \| State and municipal
Assets:
Total fixed maturity securities available for sale	4,064,854	4,445,094
Level 2 \| Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Total fixed maturity securities available for sale	1,458,512	1,299,931
Level 2 \| Fair Value, Measurements, Recurring \| Asset-backed
Assets:
Total fixed maturity securities available for sale	2,354,814	2,111,372
Level 2 \| Fair Value, Measurements, Recurring \| Corporate
Assets:
Total fixed maturity securities available for sale	3,978,189	4,389,112
Level 2 \| Fair Value, Measurements, Recurring \| Foreign government
Assets:
Total fixed maturity securities available for sale	801,540	848,497
Level 2 \| Fair Value, Measurements, Recurring \| Common stocks
Assets:
Equity securities	0	0
Level 2 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	173,518	213,600
Level 3 \| Fair Value, Measurements, Recurring
Assets:
Total fixed maturity securities available for sale	101	172
Equity securities	19,769	20,213
Arbitrage trading account	0	0
Assets, fair value disclosure	19,870	20,385
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	101	172
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	8,925	9,370
Level 3 \| Fair Value, Measurements, Recurring \| Preferred stocks
Assets:
Equity securities	$ 10,844	$ 10,843

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
-Subparagraph (a)
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Amount of debt securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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