Short-term Investments (Amortized Cost, Gross Unrealized Gains And Losses And Fair Value Of Investment Securities Available-For-Sale) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,514,275	$ 1,205,137
Unrealized Gains	2,084	2,423
Unrealized Losses	(3,687)	(2,876)
Fair Value	1,512,672	1,204,684
U.S. Treasury And Agency Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	678,771	506,094
Unrealized Gains	509	544
Unrealized Losses	(487)	(353)
Fair Value	678,793	506,285
Corporate And Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	400,683	300,994
Unrealized Gains	605	1,090
Unrealized Losses	(654)	(243)
Fair Value	400,634	301,841
Certificates of Deposit and Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	68,915	99,897
Unrealized Gains	56	62
Unrealized Losses	0	0
Fair Value	68,971	99,959
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	228,298	160,559
Unrealized Gains	109	99
Unrealized Losses	(388)	(391)
Fair Value	228,019	160,267
Municipal Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	114,018	114,888
Unrealized Gains	508	348
Unrealized Losses	(12)	(40)
Fair Value	114,514	115,196
Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,590	22,705
Unrealized Gains	297	280
Unrealized Losses	(2,146)	(1,849)
Fair Value	$ 21,741	$ 21,136