Marketable Securities (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Marketable Securities
Marketable securities, realized gain (loss)	$ 14,000	$ (139,000)	$ 90,000
Weighted average maturity of current marketable securities	7 months
Summary of Available-for-sale marketable securities
Estimated Fair Value, Current	331,192,000	474,791,000
Estimated Fair Value, Non-current	8,323,000	22,102,000
Adjusted Cost	339,440,000	527,106,000
Unrealized Gains	334,000	198,000
Unrealized Losses	(259,000)	(2,210,000)
Estimated Fair Value, available-for-sale marketable securities	339,515,000	525,094,000
Agency bond investments
Summary of Available-for-sale marketable securities
Adjusted Cost, Current	107,673,000	187,709,000
Unrealized Gains, Current	65,000	70,000
Unrealized Losses, Current		(7,000)
Estimated Fair Value, Current	107,738,000	187,772,000
Adjusted Cost	107,673,000	187,709,000
Unrealized Gains	65,000	70,000
Unrealized Losses		(7,000)
Estimated Fair Value, agency bond investments and corporate investments	107,738,000	187,772,000
Corporate debt investments
Summary of Available-for-sale marketable securities
Adjusted Cost, Current	223,192,000	315,522,000
Unrealized Gains, Current	269,000	128,000
Unrealized Losses, Current	(7,000)	(430,000)
Estimated Fair Value, Current	223,454,000	315,220,000
Adjusted Cost, Non-current	8,575,000	23,875,000
Unrealized Losses, Non-current	(252,000)	(1,773,000)
Estimated Fair Value, Non-current	8,323,000	22,102,000
Adjusted Cost	231,767,000	339,397,000
Unrealized Gains	269,000	128,000
Unrealized Losses	(259,000)	(2,203,000)
Estimated Fair Value, agency bond investments and corporate investments	$ 231,777,000	$ 337,322,000