Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	$ 11,515,524	$ 11,841,005
Equity securities available for sale	283,338	376,022
Arbitrage trading account	522,128	329,077
Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	11,515,524	11,841,005
Equity securities available for sale	283,338	376,022
Arbitrage trading account	522,128	329,077
Total assets	12,320,990	12,546,104
Securities sold but not yet purchased	162,278	121,487
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	0	0
Equity securities available for sale	159,537	280,658
Arbitrage trading account	192,281	233,603
Total assets	351,818	514,261
Securities sold but not yet purchased	162,126	114,909
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	11,472,660	11,781,940
Equity securities available for sale	118,811	93,335
Arbitrage trading account	328,067	94,546
Total assets	11,919,538	11,969,821
Securities sold but not yet purchased	152	6,558
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	42,864	59,065
Equity securities available for sale	4,990	2,029
Arbitrage trading account	1,780	928
Total assets	49,634	62,022
Securities sold but not yet purchased	0	20
Recurring | U.S. government and government agency | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	884,859	898,463
Recurring | U.S. government and government agency | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	0	0
Recurring | U.S. government and government agency | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	884,859	898,463
Recurring | U.S. government and government agency | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	0	0
Recurring | State and municipal | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	4,218,284	4,768,519
Recurring | State and municipal | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	0	0
Recurring | State and municipal | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	4,218,284	4,768,519
Recurring | State and municipal | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	0	0
Recurring | Mortgage-backed securities | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	1,329,783	1,715,697
Recurring | Mortgage-backed securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	0	0
Recurring | Mortgage-backed securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	1,329,783	1,715,697
Recurring | Mortgage-backed securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	0	0
Recurring | Corporate | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	4,202,741	3,601,648
Recurring | Corporate | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	0	0
Recurring | Corporate | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	4,159,877	3,542,583
Recurring | Corporate | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	42,864	59,065
Recurring | Foreign government | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	879,857	856,678
Recurring | Foreign government | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	0	0
Recurring | Foreign government | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	879,857	856,678
Recurring | Foreign government | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities available for sale	0	0
Recurring | Common stocks | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities available for sale	160,775	282,066
Recurring | Common stocks | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities available for sale	159,537	280,658
Recurring | Common stocks | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities available for sale	0	0
Recurring | Common stocks | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities available for sale	1,238	1,408
Recurring | Preferred stocks | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities available for sale	122,563	93,956
Recurring | Preferred stocks | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities available for sale	0	0
Recurring | Preferred stocks | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities available for sale	118,811	93,335
Recurring | Preferred stocks | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities available for sale	$ 3,752	$ 621