Fair Value Measurements (Assets and Liabilities Measured At Fair Value, On a Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	$ 12,294,504	$ 12,603,983
Loans receivable	153,513	170,991
Arbitrage trading account	830,212	450,648
Level 1 | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	0	0
Loans receivable	50,172	65,605
Assets, fair value disclosure	437,747	360,652
Arbitrage trading account	387,575	295,047
Securities sold but not yet purchased	155,428	106,074
Level 1 | Fair Value, Measurements, Recurring [Member] | U.S. government and government agency
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	0	0
Level 1 | Fair Value, Measurements, Recurring [Member] | State and municipal
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	0	0
Level 1 | Fair Value, Measurements, Recurring [Member] | Mortgage-backed securities
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	0	0
Level 1 | Fair Value, Measurements, Recurring [Member] | Corporate
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	0	0
Level 1 | Fair Value, Measurements, Recurring [Member] | Foreign government
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	0	0
Level 1 | Fair Value, Measurements, Recurring [Member] | Common stocks
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Loans receivable	50,172	65,605
Level 1 | Fair Value, Measurements, Recurring [Member] | Preferred stocks
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Loans receivable	0	0
Level 2 | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	12,294,350	12,583,218
Loans receivable	90,647	90,932
Assets, fair value disclosure	12,826,577	12,829,031
Arbitrage trading account	441,580	154,881
Securities sold but not yet purchased	56	5
Level 2 | Fair Value, Measurements, Recurring [Member] | U.S. government and government agency
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	715,367	803,388
Level 2 | Fair Value, Measurements, Recurring [Member] | State and municipal
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	4,298,851	4,362,996
Level 2 | Fair Value, Measurements, Recurring [Member] | Mortgage-backed securities
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	1,169,283	1,295,636
Level 2 | Fair Value, Measurements, Recurring [Member] | Corporate
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	5,143,699	5,179,372
Level 2 | Fair Value, Measurements, Recurring [Member] | Foreign government
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	967,150	941,826
Level 2 | Fair Value, Measurements, Recurring [Member] | Common stocks
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Loans receivable	0	0
Level 2 | Fair Value, Measurements, Recurring [Member] | Preferred stocks
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Loans receivable	90,647	90,932
Level 3 | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	154	20,765
Loans receivable	12,694	14,454
Assets, fair value disclosure	13,905	35,939
Arbitrage trading account	1,057	720
Securities sold but not yet purchased	0	0
Level 3 | Fair Value, Measurements, Recurring [Member] | U.S. government and government agency
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	0	0
Level 3 | Fair Value, Measurements, Recurring [Member] | State and municipal
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	0	0
Level 3 | Fair Value, Measurements, Recurring [Member] | Mortgage-backed securities
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	0	0
Level 3 | Fair Value, Measurements, Recurring [Member] | Corporate
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	154	20,765
Level 3 | Fair Value, Measurements, Recurring [Member] | Foreign government
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	0	0
Level 3 | Fair Value, Measurements, Recurring [Member] | Common stocks
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Loans receivable	9,019	10,741
Level 3 | Fair Value, Measurements, Recurring [Member] | Preferred stocks
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Loans receivable	3,675	3,713
Fair Value | Fair Value, Measurements, Recurring [Member]
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	12,294,504	12,603,983
Loans receivable	153,513	170,991
Assets, fair value disclosure	13,278,229	13,225,622
Arbitrage trading account	830,212	450,648
Securities sold but not yet purchased	155,484	106,079
Fair Value | Fair Value, Measurements, Recurring [Member] | U.S. government and government agency
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	715,367	803,388
Fair Value | Fair Value, Measurements, Recurring [Member] | State and municipal
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	4,298,851	4,362,996
Fair Value | Fair Value, Measurements, Recurring [Member] | Mortgage-backed securities
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	1,169,283	1,295,636
Fair Value | Fair Value, Measurements, Recurring [Member] | Corporate
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	5,143,853	5,200,137
Fair Value | Fair Value, Measurements, Recurring [Member] | Foreign government
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Fixed maturity securities	967,150	941,826
Fair Value | Fair Value, Measurements, Recurring [Member] | Common stocks
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Loans receivable	59,191	76,346
Fair Value | Fair Value, Measurements, Recurring [Member] | Preferred stocks
Fair Value Measurements, Recurring and Nonrecurring Basis [Line Items]
Loans receivable	$ 94,322	$ 94,645