Fair Value Measurements (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Oct. 31, 2011
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	$ 354,958,000	$ 325,159,000
Convertible senior notes
Convertible Debt [Abstract]
Total convertible senior notes - par	400,000,000	400,000,000	400,000,000
Interest rate, stated percentage	1.875%		1.875%
Fair value of the outstanding notes	843,200,000	375,500,000
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of marketable securities held	1	1
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	0	0
Unrestricted
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	349,049,000	319,219,000
Measured on a recurring basis | Fair Value
Available-for-sale marketable securities:
Total fair value of available-for-sale marketable securities	354,958,000	325,159,000
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	3,393,000	3,019,000
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	351,565,000	322,140,000
Measured on a recurring basis | Unrestricted | Fair Value
Available-for-sale marketable securities:
U.S. government and government agency obligations	328,146,000	284,819,000
U.S. government guaranteed corporate bonds		15,317,000
U S government guaranteed collateralized mortgage obligations		623,000
Municipal bonds	17,510,000	15,326,000
Mortgage-backed securities		115,000
Equity security	3,393,000	3,019,000
Total fair value of available-for-sale marketable securities	349,049,000	319,219,000
Measured on a recurring basis | Unrestricted | Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale marketable securities:
Equity security	3,393,000	3,019,000
Total fair value of available-for-sale marketable securities	3,393,000	3,019,000
Measured on a recurring basis | Unrestricted | Significant Other Observable Inputs (Level 2)
Available-for-sale marketable securities:
U.S. government and government agency obligations	328,146,000	284,819,000
U.S. government guaranteed corporate bonds		15,317,000
U S government guaranteed collateralized mortgage obligations		623,000
Municipal bonds	17,510,000	15,326,000
Mortgage-backed securities		115,000
Total fair value of available-for-sale marketable securities	345,656,000	316,200,000
Measured on a recurring basis | Restricted | Fair Value
Available-for-sale marketable securities:
U.S. government and government agency obligations	5,909,000	5,940,000
Measured on a recurring basis | Restricted | Significant Other Observable Inputs (Level 2)
Available-for-sale marketable securities:
U.S. government and government agency obligations	$ 5,909,000	$ 5,940,000