Fair Value Measurements (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 0	$ 0	$ 0	$ 0
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	988,508,000		988,508,000		$ 868,283,000
Convertible Debt [Abstract]
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount		91,400,000		91,400,000
Convertible senior notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible Debt	$ 200,000		$ 200,000		$ 11,200,000
Convertible Debt [Abstract]
Interest rate, stated percentage	1.875%		1.875%		1.875%
Fair value of the outstanding notes	$ 1,100,000		$ 1,100,000		$ 72,800,000
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other than Temporary Impairment Losses, Investments	0	0	0	0
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities, Unrealized Gain (Loss)	0	$ 0	0	$ 0
Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	17,613,000		17,613,000		31,467,000
Measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	970,895,000		970,895,000		836,816,000
Unrestricted [Member]
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	935,430,000		935,430,000		868,283,000
Unrestricted [Member] | Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale marketable securities:
U.S. government and government agency obligations	0		0		0
Corporate bonds	0		0		0
Municipal bonds	0		0		0
Commercial Paper	0		0
Certificates of Deposit, at Carrying Value	0		0
Equity securities	17,613,000		17,613,000		31,467,000
Total fair value of available-for-sale marketable securities	17,613,000		17,613,000		31,467,000
Unrestricted [Member] | Measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Available-for-sale marketable securities:
U.S. government and government agency obligations	63,132,000		63,132,000		51,209,000
Corporate bonds	804,303,000		804,303,000		767,683,000
Municipal bonds	12,733,000		12,733,000		17,924,000
Commercial Paper	29,635,000		29,635,000
Certificates of Deposit, at Carrying Value	8,014,000		8,014,000
Equity securities	0		0		0
Total fair value of available-for-sale marketable securities	917,817,000		917,817,000		836,816,000
Restricted [Member]
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	53,078,000		53,078,000
Restricted [Member] | Measured on a recurring basis
Available-for-sale marketable securities:
Equity securities	53,078,000		53,078,000
Restricted [Member] | Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale marketable securities:
Equity securities	0		0
Estimate of Fair Value Measurement [Member] | Measured on a recurring basis
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	988,508,000		988,508,000		868,283,000
Estimate of Fair Value Measurement [Member] | Unrestricted [Member] | Measured on a recurring basis
Available-for-sale marketable securities:
U.S. government and government agency obligations	63,132,000		63,132,000		51,209,000
Corporate bonds	804,303,000		804,303,000		767,683,000
Municipal bonds	12,733,000		12,733,000		17,924,000
Commercial Paper	29,635,000		29,635,000
Certificates of Deposit, at Carrying Value	8,014,000		8,014,000
Equity securities	17,613,000		17,613,000		31,467,000
Total fair value of available-for-sale marketable securities	935,430,000		935,430,000		$ 868,283,000
Estimate of Fair Value Measurement [Member] | Restricted [Member] | Measured on a recurring basis
Available-for-sale marketable securities:
Equity securities	$ 53,078,000		$ 53,078,000