Marketable Securities (Details) (USD $)	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains	$ 500,000	$ 500,000
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	326,859,000	326,859,000	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)
Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	331,454,000	331,454,000	349,088,000
Unrealized Gains	59,000	59,000	661,000
Unrealized (Losses)	(2,354,000)	(2,354,000)	(700,000)
Available-for-sale Securities, Fair Value Disclosure	329,159,000	329,159,000	349,049,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year	155,831,000	155,831,000	77,819,000
Maturities after one year through five years	171,028,000	171,028,000	267,837,000
Total	326,859,000	326,859,000	345,656,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	223,844,000	223,844,000	62,248,000
Fair Value - 12 Months or Greater	2,300,000	2,300,000	3,393,000
Fair Value - Total	226,144,000	226,144,000	65,641,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(610,000)	(610,000)	(49,000)
Unrealized Loss - 12 Months or Greater	(1,744,000)	(1,744,000)	(651,000)
Unrealized Loss - Total	(2,354,000)	(2,354,000)	(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	85,662,000	85,662,000	327,502,000
Unrealized Gains	52,000	52,000	661,000
Unrealized (Losses)	(309,000)	(309,000)	(17,000)
Available-for-sale Securities, Fair Value Disclosure	85,405,000	85,405,000	328,146,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	69,106,000	69,106,000	44,738,000
Fair Value - Total	69,106,000	69,106,000	44,738,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(309,000)	(309,000)	(17,000)
Unrealized Loss - Total	(309,000)	(309,000)	(17,000)
Corporate bond | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	157,593,000	157,593,000
Unrealized Gains	6,000	6,000
Unrealized (Losses)	(271,000)	(271,000)
Available-for-sale Securities, Fair Value Disclosure	157,328,000	157,328,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	142,244,000	142,244,000
Fair Value - Total	142,244,000	142,244,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(271,000)	(271,000)
Unrealized Loss - Total	(271,000)	(271,000)
Commercial paper | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	62,159,000	62,159,000
Unrealized Gains
Unrealized (Losses)
Available-for-sale Securities, Fair Value Disclosure	62,159,000	62,159,000
Municipal bonds | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	17,172,000	17,172,000	17,542,000
Unrealized Gains	1,000	1,000
Unrealized (Losses)	(22,000)	(22,000)	(32,000)
Available-for-sale Securities, Fair Value Disclosure	17,151,000	17,151,000	17,510,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	7,678,000	7,678,000	17,510,000
Fair Value - Total	7,678,000	7,678,000	17,510,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(22,000)	(22,000)	(32,000)
Unrealized Loss - Total	(22,000)	(22,000)	(32,000)
Foreign Government Debt Securities [Member] | Unrestricted
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	4,824,000	4,824,000
Unrealized Gains
Unrealized (Losses)	(8,000)	(8,000)
Available-for-sale Securities, Fair Value Disclosure	4,816,000	4,816,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months	4,816,000	4,816,000
Fair Value - Total	4,816,000	4,816,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months	(8,000)	(8,000)
Unrealized Loss - Total	(8,000)	(8,000)
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)	(1,744,000)	(1,744,000)	(651,000)
Available-for-sale Securities, Fair Value Disclosure	2,300,000	2,300,000	3,393,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months
Fair Value - 12 Months or Greater	2,300,000	2,300,000	3,393,000
Fair Value - Total	2,300,000	2,300,000	3,393,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months
Unrealized Loss - 12 Months or Greater	(1,744,000)	(1,744,000)	(651,000)
Unrealized Loss - Total	(1,744,000)	(1,744,000)	(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)