Marketable Securities (Details) (USD $)	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Marketable Securities
Weighted average maturity of current marketable securities	12 months
Summary of Available-for-sale marketable securities
Estimated Fair Value, Current	$ 427,388,000	$ 331,192,000
Estimated Fair Value, Non-current	5,451,000	8,323,000
Adjusted Cost	433,480,000	339,440,000
Unrealized Gains	139,000	334,000
Unrealized Losses	(779,000)	(259,000)
Estimated Fair Value, available-for-sale marketable securities	432,840,000	339,515,000
Amount of auction rate securities in investment portfolio	5,700,000
Government/ agency bond investments
Summary of Available-for-sale marketable securities
Adjusted Cost, Current	159,648,000	107,673,000
Unrealized Gains, Current	51,000	65,000
Unrealized Losses, Current	(110,000)
Estimated Fair Value, Current	159,589,000	107,738,000
Adjusted Cost	159,648,000	107,673,000
Unrealized Gains	51,000	65,000
Unrealized Losses	(110,000)
Estimated Fair Value, agency bond investments and corporate investments	159,589,000	107,738,000
Corporate debt and other investments
Summary of Available-for-sale marketable securities
Adjusted Cost, Current	268,157,000	223,192,000
Unrealized Gains, Current	88,000	269,000
Unrealized Losses, Current	(445,000)	(7,000)
Estimated Fair Value, Current	267,800,000	223,454,000
Adjusted Cost, Non-current	5,675,000	8,575,000
Unrealized Losses, Non-current	(224,000)	(252,000)
Estimated Fair Value, Non-current	5,451,000	8,323,000
Adjusted Cost	273,832,000	231,767,000
Unrealized Gains	88,000	269,000
Unrealized Losses	(669,000)	(259,000)
Estimated Fair Value, agency bond investments and corporate investments	$ 273,251,000	$ 231,777,000