Marketable Securities (Details) (USD $)	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains	$ 500,000	$ 1,000,000
Amortized Cost Basis			354,990,000
Unrealized Gains			670,000
Unrealized (Losses)			(702,000)
Available-for-sale Securities, Fair Value Disclosure			354,958,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Total	509,733,000	509,733,000	351,565,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months			63,442,000
Fair Value - 12 Months or Greater			3,393,000
Fair Value - Total			66,835,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months			(51,000)
Unrealized Loss - 12 Months or Greater			(651,000)
Unrealized Loss - Total			(702,000)
Available-for-sale Securities, Gross Realized Losses	0	0
Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	513,693,000	513,693,000	349,088,000
Unrealized Gains	291,000	291,000	661,000
Unrealized (Losses)	(2,009,000)	(2,009,000)	(700,000)
Available-for-sale Securities, Fair Value Disclosure	511,975,000	511,975,000	349,049,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	182,112,000	182,112,000	77,819,000
Maturities after one year through five years	322,314,000	322,314,000	267,837,000
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	4,132,000	4,132,000
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	1,175,000	1,175,000
Total	509,733,000	509,733,000	345,656,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	230,619,000	230,619,000	62,248,000
Fair Value - 12 Months or Greater	2,242,000	2,242,000	3,393,000
Fair Value - Total	232,861,000	232,861,000	65,641,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(207,000)	(207,000)	(49,000)
Unrealized Loss - 12 Months or Greater	(1,802,000)	(1,802,000)	(651,000)
Unrealized Loss - Total	(2,009,000)	(2,009,000)	(700,000)
Restricted
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year			2,781,000
Maturities after one year through five years			3,128,000
Total			5,909,000
U.S. government and government agency obligations | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	106,788,000	106,788,000	327,502,000
Unrealized Gains	55,000	55,000	661,000
Unrealized (Losses)	(54,000)	(54,000)	(17,000)
Available-for-sale Securities, Fair Value Disclosure	106,789,000	106,789,000	328,146,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	57,612,000	57,612,000	44,738,000
Fair Value - Total	57,612,000	57,612,000	44,738,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(54,000)	(54,000)	(17,000)
Unrealized Loss - Total	(54,000)	(54,000)	(17,000)
Corporate bond | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	303,206,000	303,206,000
Unrealized Gains	128,000	128,000
Unrealized (Losses)	(140,000)	(140,000)
Available-for-sale Securities, Fair Value Disclosure	303,194,000	303,194,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	157,200,000	157,200,000
Fair Value - Total	157,200,000	157,200,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(140,000)	(140,000)
Unrealized Loss - Total	(140,000)	(140,000)
Commercial paper | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	57,046,000	57,046,000
Unrealized Gains	72,000	72,000
Unrealized (Losses)
Available-for-sale Securities, Fair Value Disclosure	57,118,000	57,118,000
Municipal bonds | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	26,798,000	26,798,000	17,542,000
Unrealized Gains	34,000	34,000
Unrealized (Losses)	(9,000)	(9,000)	(32,000)
Available-for-sale Securities, Fair Value Disclosure	26,823,000	26,823,000	17,510,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	7,504,000	7,504,000	17,510,000
Fair Value - Total	7,504,000	7,504,000	17,510,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(9,000)	(9,000)	(32,000)
Unrealized Loss - Total	(9,000)	(9,000)	(32,000)
International government agency obligations | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	8,308,000	8,308,000
Unrealized Gains
Unrealized (Losses)	(4,000)	(4,000)
Available-for-sale Securities, Fair Value Disclosure	8,304,000	8,304,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	8,303,000	8,303,000
Fair Value - Total	8,303,000	8,303,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(4,000)	(4,000)
Unrealized Loss - Total	(4,000)	(4,000)
Certificates of deposit | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	7,503,000	7,503,000
Unrealized Gains	2,000	2,000
Unrealized (Losses)
Available-for-sale Securities, Fair Value Disclosure	7,505,000	7,505,000
Equity securities | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	4,044,000	4,044,000	4,044,000
Unrealized Gains
Unrealized (Losses)	(1,802,000)	(1,802,000)	(651,000)
Available-for-sale Securities, Fair Value Disclosure	2,242,000	2,242,000	3,393,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months
Fair Value - 12 Months or Greater	2,242,000	2,242,000	3,393,000
Fair Value - Total	2,242,000	2,242,000	3,393,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months
Unrealized Loss - 12 Months or Greater	(1,802,000)	(1,802,000)	(651,000)
Unrealized Loss - Total	(1,802,000)	(1,802,000)	(651,000)
U.S. government obligations | Restricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis			5,902,000
Unrealized Gains			9,000
Unrealized (Losses)			(2,000)
Available-for-sale Securities, Fair Value Disclosure			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)