Investments (Details 2) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Realized Gains and Losses on Investments
Realized (loss) gain from sale of securities related to ARS holdings	$ (1,471,000)	$ (209,000)	$ (1,471,000)	$ (208,000)
Summary of Fair Value of Investments with Gross Unrealized Losses
Fair value of investments in continuous unrealized loss position for less than 12 months	74,301,000		74,301,000		48,929,000
Gross unrealized losses on investments in continuous unrealized loss position for less than 12 months	(106,000)		(106,000)		(174,000)
Fair value of investments in continuous unrealized loss position for 12 months or greater	937,000		937,000		24,024,000
Gross unrealized losses on investments in continuous unrealized loss position for 12 months or greater	(15,000)		(15,000)		(2,377,000)
Fair value of investments in continuous unrealized loss position	75,238,000		75,238,000		72,953,000
Gross unrealized losses on investments in continuous unrealized loss position	(121,000)		(121,000)		(2,551,000)
Corporate debt securities
Summary of Fair Value of Investments with Gross Unrealized Losses
Fair value of investments in continuous unrealized loss position for less than 12 months	58,433,000		58,433,000		34,097,000
Gross unrealized losses on investments in continuous unrealized loss position for less than 12 months	(92,000)		(92,000)		(161,000)
Fair value of investments in continuous unrealized loss position for 12 months or greater	937,000		937,000		4,124,000
Gross unrealized losses on investments in continuous unrealized loss position for 12 months or greater	(15,000)		(15,000)		(4,000)
Fair value of investments in continuous unrealized loss position	59,370,000		59,370,000		38,221,000
Gross unrealized losses on investments in continuous unrealized loss position	(107,000)		(107,000)		(165,000)
U.S. treasury and government agency securities
Summary of Fair Value of Investments with Gross Unrealized Losses
Fair value of investments in continuous unrealized loss position for less than 12 months	8,391,000		8,391,000		8,841,000
Gross unrealized losses on investments in continuous unrealized loss position for less than 12 months	(4,000)		(4,000)		(7,000)
Fair value of investments in continuous unrealized loss position	8,391,000		8,391,000		8,841,000
Gross unrealized losses on investments in continuous unrealized loss position	(4,000)		(4,000)		(7,000)
Commercial paper
Summary of Fair Value of Investments with Gross Unrealized Losses
Fair value of investments in continuous unrealized loss position for less than 12 months	7,477,000		7,477,000		5,991,000
Gross unrealized losses on investments in continuous unrealized loss position for less than 12 months	(10,000)		(10,000)		(6,000)
Fair value of investments in continuous unrealized loss position	7,477,000		7,477,000		5,991,000
Gross unrealized losses on investments in continuous unrealized loss position	(10,000)		(10,000)		(6,000)
Auction rate securities
Realized Gains and Losses on Investments
Aggregate par value of auction rate securities sold during the period	19,800,000
Proceeds from the sale of ARS holdings	18,300,000
Realized (loss) gain from sale of securities related to ARS holdings	1,500,000		1,500,000
Summary of Fair Value of Investments with Gross Unrealized Losses
Fair value of investments in continuous unrealized loss position for 12 months or greater					19,900,000
Gross unrealized losses on investments in continuous unrealized loss position for 12 months or greater					(2,373,000)
Fair value of investments in continuous unrealized loss position					19,900,000
Gross unrealized losses on investments in continuous unrealized loss position					$ (2,373,000)