Marketable Securities (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments		$ 0	$ 0
Available-for-sale Securities, Gross Realized Gains	1,000,000
Amortized Cost Basis	548,322,000	354,990,000
Unrealized Gains	392,000	670,000
Unrealized (Losses)	(447,000)	(702,000)
Total fair value of available-for-sale marketable securities	548,267,000	354,958,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total	547,101,000	351,565,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	239,812,000	63,442,000
Fair Value - 12 Months or Greater	0	3,393,000
Fair Value - Total	239,812,000	66,835,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(447,000)	(51,000)
Unrealized Loss - 12 Months or Greater	0	(651,000)
Unrealized Loss - Total	(447,000)	(702,000)
Available-for-sale Securities, Gross Realized Losses	0
Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		349,088,000
Unrealized Gains		661,000
Unrealized (Losses)		(700,000)
Total fair value of available-for-sale marketable securities		349,049,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	158,376,000	77,819,000
Maturities after one year through five years	383,410,000	267,837,000
Maturities after five years through ten years	4,138,000	0
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	1,177,000	0
Total	547,101,000	345,656,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		62,248,000
Fair Value - 12 Months or Greater		3,393,000
Fair Value - Total		65,641,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months		(49,000)
Unrealized Loss - 12 Months or Greater		(651,000)
Unrealized Loss - Total		(700,000)
Restricted
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	0	2,781,000
Maturities after one year through five years	0	3,128,000
Total	0	5,909,000
U.S. government and government agency obligations | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	107,493,000	327,502,000
Unrealized Gains	55,000	661,000
Unrealized (Losses)	(27,000)	(17,000)
Total fair value of available-for-sale marketable securities	107,521,000	328,146,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	49,241,000	44,738,000
Fair Value - Total	49,241,000	44,738,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(27,000)	(17,000)
Unrealized Loss - Total	(27,000)	(17,000)
U.S. government and government agency obligations | Restricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		5,902,000
Unrealized Gains		9,000
Unrealized (Losses)		(2,000)
Total fair value of available-for-sale marketable securities		5,909,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		1,194,000
Fair Value - Total		1,194,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months		(2,000)
Unrealized Loss - Total		(2,000)
Corporate bonds | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	369,321,000
Unrealized Gains	233,000
Unrealized (Losses)	(361,000)
Total fair value of available-for-sale marketable securities	369,193,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	176,140,000
Fair Value - Total	176,140,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(361,000)
Unrealized Loss - Total	(361,000)
Commercial Paper | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	23,891,000
Unrealized Gains	53,000
Unrealized (Losses)	0
Total fair value of available-for-sale marketable securities	23,944,000
Municipal bonds | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	36,935,000	17,542,000
Unrealized Gains	45,000	0
Unrealized (Losses)	(59,000)	(32,000)
Total fair value of available-for-sale marketable securities	36,921,000	17,510,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	14,431,000	17,510,000
Fair Value - Total	14,431,000	17,510,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(59,000)	(32,000)
Unrealized Loss - Total	(59,000)	(32,000)
International government agency obligations | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	2,007,000
Unrealized Gains	1,000
Unrealized (Losses)	0
Total fair value of available-for-sale marketable securities	2,008,000
Certificates of Deposit | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	7,509,000
Unrealized Gains	5,000
Unrealized (Losses)	0
Total fair value of available-for-sale marketable securities	7,514,000
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments	2,900,000
Equity securities | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	1,166,000	4,044,000
Unrealized Gains	0	0
Unrealized (Losses)	0	(651,000)
Total fair value of available-for-sale marketable securities	1,166,000	3,393,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - 12 Months or Greater		3,393,000
Fair Value - Total		3,393,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - 12 Months or Greater		(651,000)
Unrealized Loss - Total		$ (651,000)