Cash, Cash Equivalents and Investments (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	$ 46,897	$ 46,186
Fair value of available-for-sale securities, continuous loss position for twelve month or longer	17,729	17,477
Total fair value of available-for-sale securities, continuous loss position	64,626	63,663
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(61)	(392)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(1,446)	(1,748)
Available-for-sale securities, total gross unrealized losses	(1,507)	(2,140)
Cost
Due in one year or less, Cost	139,344
Due after one year through ten years, Cost	110,933
Due after ten years, Cost	58,635
Total Cost	308,912
Fair Value
Due in one year or less, Fair Value	139,480
Due after one year through ten years, Fair Value	111,146
Due after ten years, Fair Value	57,189
Total Fair Value	307,815
Corporate bonds
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	22,814	25,438
Total fair value of available-for-sale securities, continuous loss position	22,814	25,438
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(43)	(363)
Available-for-sale securities, total gross unrealized losses	(43)	(363)
Auction rate securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for twelve month or longer	17,729	17,477
Total fair value of available-for-sale securities, continuous loss position	17,729	17,477
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(1,446)	(1,748)
Available-for-sale securities, total gross unrealized losses	(1,446)	(1,748)
Municipal bonds
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	10,367	10,437
Total fair value of available-for-sale securities, continuous loss position	10,367	10,437
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(4)	(12)
Available-for-sale securities, total gross unrealized losses	(4)	(12)
U.S. government agency
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	12,639	5,772
Total fair value of available-for-sale securities, continuous loss position	12,639	5,772
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(13)	(12)
Available-for-sale securities, total gross unrealized losses	(13)	(12)
Asset-backed securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	1,077	4,539
Total fair value of available-for-sale securities, continuous loss position	1,077	4,539
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(1)	(5)
Available-for-sale securities, total gross unrealized losses	$ (1)	$ (5)