Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	$ 13,975,889	$ 12,889,095
Available-for-sale investments, gross unrealized holding gains	2,858,321	2,318,005
Available-for-sale investments, gross unrealized holding losses	(39,610)	(92,014)
Available-for-sale investments, unrealized other-than-temporary impairment losses	(3,911)	(3,882)
Available-for-sale investments, estimated fair value	16,790,689	15,111,204
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	674,801	695,652
Available-for-sale investments, gross unrealized holding gains	25,489	9,836
Available-for-sale investments, gross unrealized holding losses	(9)	(4,781)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	700,281	700,707
Obligations Of States, Municipalities And Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	4,193,114	3,817,136
Available-for-sale investments, gross unrealized holding gains	370,264	204,302
Available-for-sale investments, gross unrealized holding losses	(2,329)	(8,225)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	4,561,049	4,013,213
Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,349,640	1,302,329
Available-for-sale investments, gross unrealized holding gains	233,884	115,809
Available-for-sale investments, gross unrealized holding losses	(305)	(1,681)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	1,583,219	1,416,457
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	975,606	657,670
Available-for-sale investments, gross unrealized holding gains	48,531	6,867
Available-for-sale investments, gross unrealized holding losses	(533)	(4,999)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	1,023,604	659,538
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	805,631	837,964
Available-for-sale investments, gross unrealized holding gains	57,100	22,563
Available-for-sale investments, gross unrealized holding losses	(529)	(4,022)
Available-for-sale investments, unrealized other-than-temporary impairment losses	(2,258)	(2,258)
Available-for-sale investments, estimated fair value	859,944	854,247
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	28,232	36,462
Available-for-sale investments, gross unrealized holding gains	127	15
Available-for-sale investments, gross unrealized holding losses	(17)	(406)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	28,342	36,071
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,563,313	1,690,945
Available-for-sale investments, gross unrealized holding gains	82,823	41,123
Available-for-sale investments, gross unrealized holding losses	(1,293)	(16,209)
Available-for-sale investments, unrealized other-than-temporary impairment losses	(1,653)	(1,624)
Available-for-sale investments, estimated fair value	1,643,190	1,714,235
Total Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	9,590,337	9,038,158
Available-for-sale investments, gross unrealized holding gains	818,218	400,515
Available-for-sale investments, gross unrealized holding losses	(5,015)	(40,323)
Available-for-sale investments, unrealized other-than-temporary impairment losses	(3,911)	(3,882)
Available-for-sale investments, estimated fair value	10,399,629	9,394,468
Insurance, Banks And Other Financial Institutions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	835,144	651,002
Available-for-sale investments, gross unrealized holding gains	769,505	690,271
Available-for-sale investments, gross unrealized holding losses	(12,641)	(6,551)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	1,592,008	1,334,722
Industrial, Consumer And All Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,555,040	1,557,832
Available-for-sale investments, gross unrealized holding gains	1,270,474	1,227,052
Available-for-sale investments, gross unrealized holding losses	(21,953)	(45,131)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	2,803,561	2,739,753
Total Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	2,390,184	2,208,834
Available-for-sale investments, gross unrealized holding gains	2,039,979	1,917,323
Available-for-sale investments, gross unrealized holding losses	(34,594)	(51,682)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	4,395,569	4,074,475
Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments, amortized cost	1,995,368	1,642,103
Available-for-sale investments, gross unrealized holding gains	124	167
Available-for-sale investments, gross unrealized holding losses	(1)	(9)
Available-for-sale investments, unrealized other-than-temporary impairment losses	0	0
Available-for-sale investments, estimated fair value	$ 1,995,491	$ 1,642,261