Fair Value Measurements (Details) (USD $)					3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Convertible senior notes	Dec. 31, 2013 Convertible senior notes	Mar. 31, 2014 Significant Other Observable Inputs (Level 2)	Mar. 31, 2013 Significant Other Observable Inputs (Level 2)	Mar. 31, 2014 Measured on a recurring basis Fair Value	Dec. 31, 2013 Measured on a recurring basis Fair Value	Mar. 31, 2014 Measured on a recurring basis Quoted Prices in Active Markets for Identical Assets (Level 1)	Dec. 31, 2013 Measured on a recurring basis Quoted Prices in Active Markets for Identical Assets (Level 1)	Mar. 31, 2014 Measured on a recurring basis Significant Other Observable Inputs (Level 2)	Dec. 31, 2013 Measured on a recurring basis 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
Available-for-sale marketable securities:
U.S. government and government agency obligations							78,265,000	107,521,000			78,265,000	107,521,000
Corporate bonds							579,628,000	369,193,000			579,628,000	369,193,000
Commercial paper								23,944,000				23,944,000
Municipal bonds							43,401,000	36,921,000			43,401,000	36,921,000
International government agency obligations							8,217,000	2,008,000			8,217,000	2,008,000
Certificates of deposit							7,918,000	7,514,000			7,918,000	7,514,000
Equity securities							3,535,000	1,166,000	3,535,000	1,166,000
Total fair value of available-for-sale marketable securities	720,964,000	548,267,000					720,964,000	548,267,000	3,535,000	1,166,000	717,429,000	547,101,000
Convertible Debt [Abstract]
Total convertible senior notes - par			400,000,000	400,000,000
Interest rate, stated percentage			1.875%	1.875%
Fair value of the outstanding notes			$ 1,437,700,000	$ 1,327,200,000