Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
Sep. 30, 2012	Dec. 31, 2011
Marketable Securities
Weighted average maturity of current marketable securities	9 months
Summary of Available-for-sale marketable securities
Estimated Fair Value, Current	$ 325,540	$ 474,791
Estimated Fair Value, Non-current	18,239	22,102
Adjusted Cost	400,542	527,106
Unrealized Gains	481	198
Unrealized Losses	(647)	(2,210)
Estimated Fair Value, available-for-sale marketable securities	400,376	525,094
Agency bond investments
Summary of Available-for-sale marketable securities
Adjusted Cost, Current	129,711	187,709
Unrealized Gains, Current	75	70
Unrealized Losses, Current		(7)
Estimated Fair Value, Current	129,786	187,772
Adjusted Cost	129,711	187,709
Unrealized Gains	75	70
Unrealized Losses		(7)
Estimated Fair Value, agency bond investments and corporate investments	129,786	187,772
Corporate debt investments
Summary of Available-for-sale marketable securities
Adjusted Cost, Current	251,956	315,522
Unrealized Gains, Current	406	128
Unrealized Losses, Current	(11)	(430)
Estimated Fair Value, Current	252,351	315,220
Adjusted Cost, Non-current	18,875	23,875
Unrealized Losses, Non-current	(636)	(1,773)
Estimated Fair Value, Non-current	18,239	22,102
Adjusted Cost	270,831	339,397
Unrealized Gains	406	128
Unrealized Losses	(647)	(2,203)
Estimated Fair Value, agency bond investments and corporate investments	$ 270,590	$ 337,322

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Total of debt securities categorized neither as held-to-maturity nor trading which are intended be sold or mature within one year from the balance sheet date or the normal operating cycle, whichever is longer.

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