Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	$ 12,889,095	$ 13,475,614
Available-for-sale investments, gross unrealized holding gains	2,318,005	2,714,911
Available-for-sale investments, gross unrealized holding losses	(92,014)	(31,141)
Available-for-sale investments, unrealized other-than-temporary impairment losses	(3,882)	(4,077)
Available-for-sale investments, estimated fair value	15,111,204	16,155,307
U.S. Treasury Securities And Obligations Of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	695,652	662,462
Available-for-sale investments, gross unrealized holding gains	9,836	12,963
Available-for-sale investments, gross unrealized holding losses	(4,781)	(2,163)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	700,707	673,262
Obligations Of States, Municipalities And Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	3,817,136	4,075,748
Available-for-sale investments, gross unrealized holding gains	204,302	245,158
Available-for-sale investments, gross unrealized holding losses	(8,225)	(3,359)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	4,013,213	4,317,547
Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,302,329	1,458,255
Available-for-sale investments, gross unrealized holding gains	115,809	154,707
Available-for-sale investments, gross unrealized holding losses	(1,681)	(1,041)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	1,416,457	1,611,921
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	657,670	427,904
Available-for-sale investments, gross unrealized holding gains	6,867	5,325
Available-for-sale investments, gross unrealized holding losses	(4,999)	(2,602)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	659,538	430,627
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	837,964	954,263
Available-for-sale investments, gross unrealized holding gains	22,563	34,324
Available-for-sale investments, gross unrealized holding losses	(4,022)	(3,482)
Available-for-sale investments, unrealized other-than-temporary impairment losses	(2,258)	(2,258)
Available-for-sale investments, estimated fair value	854,247	982,847
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	36,462	100,073
Available-for-sale investments, gross unrealized holding gains	15	99
Available-for-sale investments, gross unrealized holding losses	(406)	(682)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	36,071	99,490
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,690,945	2,250,432
Available-for-sale investments, gross unrealized holding gains	41,123	69,016
Available-for-sale investments, gross unrealized holding losses	(16,209)	(10,441)
Available-for-sale investments, unrealized other-than-temporary impairment losses	(1,624)	(1,819)
Available-for-sale investments, estimated fair value	1,714,235	2,307,188
Total Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	9,038,158	9,929,137
Available-for-sale investments, gross unrealized holding gains	400,515	521,592
Available-for-sale investments, gross unrealized holding losses	(40,323)	(23,770)
Available-for-sale investments, unrealized other-than-temporary impairment losses	(3,882)	(4,077)
Available-for-sale investments, estimated fair value	9,394,468	10,422,882
Insurance, Banks And Other Financial Institutions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	651,002	523,739
Available-for-sale investments, gross unrealized holding gains	690,271	789,717
Available-for-sale investments, gross unrealized holding losses	(6,551)	(1,531)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	1,334,722	1,311,925
Industrial, Consumer And All Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,557,832	1,427,919
Available-for-sale investments, gross unrealized holding gains	1,227,052	1,403,566
Available-for-sale investments, gross unrealized holding losses	(45,131)	(5,834)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	2,739,753	2,825,651
Total Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	2,208,834	1,951,658
Available-for-sale investments, gross unrealized holding gains	1,917,323	2,193,283
Available-for-sale investments, gross unrealized holding losses	(51,682)	(7,365)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	4,074,475	4,137,576
Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,642,103	1,594,819
Available-for-sale investments, gross unrealized holding gains	167	36
Available-for-sale investments, gross unrealized holding losses	(9)	(6)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	$ 1,642,261	$ 1,594,849