Fair Value Measurements (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 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
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	800,175,000		$ 868,282,000
Convertible Debt [Abstract]
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount		91,400,000
Convertible senior notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible Debt	$ 10,600,000		$ 11,200,000
Convertible Debt [Abstract]
Interest rate, stated percentage	1.875%		1.875%
Fair value of the outstanding notes	$ 49,600,000		$ 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
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
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	21,415,000		31,466,000
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	778,760,000		836,816,000
Unrestricted [Member] | Measured on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale marketable securities:
Equity securities	21,415,000		31,466,000
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	50,575,000		51,209,000
Corporate bonds	712,030,000		767,683,000
Municipal bonds	16,155,000		17,924,000
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	800,175,000		868,282,000
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	50,575,000		51,209,000
Corporate bonds	712,030,000		767,683,000
Municipal bonds	16,155,000		17,924,000
Equity securities	$ 21,415,000		$ 31,466,000