Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	$ 718,366	$ 548,322
Unrealized Gains	3,113	392
Unrealized (Losses)	(515)	(447)
Total fair value of available-for-sale marketable securities	720,964	548,267
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	186,275	158,376
Maturities after one year through five years	525,961	383,410
Maturities after five years through ten years	4,042	4,138
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	1,151	1,177
Total	717,429	547,101
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	286,344	239,812
Fair Value - 12 Months or Greater	0	0
Fair Value - Total	286,344	239,812
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(515)	(447)
Unrealized Loss - 12 Months or Greater	0	0
Unrealized Loss - Total	(515)	(447)
U.S. government and government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	78,196	107,493
Unrealized Gains	82	55
Unrealized (Losses)	(13)	(27)
Total fair value of available-for-sale marketable securities	78,265	107,521
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	5,983	49,241
Fair Value - Total	5,983	49,241
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(13)	(27)
Unrealized Loss - Total	(13)	(27)
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	579,600	369,321
Unrealized Gains	517	233
Unrealized (Losses)	(489)	(361)
Total fair value of available-for-sale marketable securities	579,628	369,193
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	268,174	176,140
Fair Value - Total	268,174	176,140
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(489)	(361)
Unrealized Loss - Total	(489)	(361)
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		23,891
Unrealized Gains		53
Unrealized (Losses)		0
Total fair value of available-for-sale marketable securities		23,944
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	43,276	36,935
Unrealized Gains	138	45
Unrealized (Losses)	(13)	(59)
Total fair value of available-for-sale marketable securities	43,401	36,921
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	12,187	14,431
Fair Value - Total	12,187	14,431
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(13)	(59)
Unrealized Loss - Total	(13)	(59)
International government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	8,214	2,007
Unrealized Gains	3	1
Unrealized (Losses)	0	0
Total fair value of available-for-sale marketable securities	8,217	2,008
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	7,914	7,509
Unrealized Gains	4	5
Unrealized (Losses)	0	0
Total fair value of available-for-sale marketable securities	7,918	7,514
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	1,166	1,166
Unrealized Gains	2,369	0
Unrealized (Losses)	0	0
Total fair value of available-for-sale marketable securities	$ 3,535	$ 1,166