Marketable Securities (Details) (USD $)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains	$ 500,000		$ 1,000,000
Available for sale securities, realized loss	0		0
Amortized Cost Basis		806,130,000		548,322,000
Unrealized Gains		44,103,000		392,000
Unrealized (Losses)		(1,140,000)		(447,000)
Total fair value of available-for-sale marketable securities		849,093,000		548,267,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year		287,739,000		158,376,000
Maturities after one year through five years		496,700,000		383,410,000
Maturities after five years through ten years		5,124,000		4,138,000
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value		0		1,177,000
Total		789,563,000		547,101,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		355,738,000		239,812,000
Fair Value - 12 Months or Greater		0		0
Fair Value - Total		355,738,000		239,812,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months		(1,140,000)		(447,000)
Unrealized Loss - 12 Months or Greater		0		0
Unrealized Loss - Total		(1,140,000)		(447,000)
U.S. government and government agency obligations
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		1,401,000		49,241,000
Fair Value - Total		1,401,000		49,241,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months		(1,000)		(27,000)
Unrealized Loss - Total		(1,000)		(27,000)
Corporate bonds
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		343,835,000		176,140,000
Fair Value - Total		343,835,000		176,140,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months		(1,131,000)		(361,000)
Unrealized Loss - Total		(1,131,000)		(361,000)
Municipal bonds
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		10,502,000		14,431,000
Fair Value - Total		10,502,000		14,431,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months		(8,000)		(59,000)
Unrealized Loss - Total		(8,000)		(59,000)
Unrestricted [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		791,130,000
Unrealized Gains		4,672,000
Unrealized (Losses)		(1,140,000)
Total fair value of available-for-sale marketable securities		794,662,000
Unrestricted [Member] | U.S. government and government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		51,059,000		107,493,000
Unrealized Gains		71,000		55,000
Unrealized (Losses)		(1,000)		(27,000)
Total fair value of available-for-sale marketable securities		51,129,000		107,521,000
Unrestricted [Member] | Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		696,149,000		369,321,000
Unrealized Gains		574,000		233,000
Unrealized (Losses)		(1,131,000)		(361,000)
Total fair value of available-for-sale marketable securities		695,592,000		369,193,000
Unrestricted [Member] | Commercial Paper
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
Unrestricted [Member] | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		42,756,000		36,935,000
Unrealized Gains		94,000		45,000
Unrealized (Losses)		(8,000)		(59,000)
Total fair value of available-for-sale marketable securities		42,842,000		36,921,000
Unrestricted [Member] | International government agency obligations [Member]
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
Unrestricted [Member] | Certificates of Deposit
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
Unrestricted [Member] | Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		1,166,000		1,166,000
Unrealized Gains		3,933,000		0
Unrealized (Losses)		0		0
Total fair value of available-for-sale marketable securities		5,099,000		1,166,000
Restricted [Member] | Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		15,000,000
Unrealized Gains		39,431,000
Unrealized (Losses)		0
Total fair value of available-for-sale marketable securities		$ 54,431,000