Investments (Details 2) (USD $)	3 Months Ended							12 Months Ended									1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Corporate debt securities	Dec. 31, 2011 Corporate debt securities	Dec. 31, 2012 U.S. treasury and government agency securities	Dec. 31, 2011 U.S. treasury and government agency securities	Dec. 31, 2012 Commercial paper	Dec. 31, 2011 Commercial paper	Jun. 30, 2012 Auction rate securities	Jun. 30, 2012 Auction rate securities	Dec. 31, 2012 Auction rate securities	Dec. 31, 2011 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	3,000	2,000	(1,471,000)	1,000	14,000	(209,000)	1,000	(1,466,000)	(193,000)	408,000								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 less than 12 months	47,299,000			48,929,000				47,299,000	48,929,000		37,036,000	34,097,000	6,271,000	8,841,000	3,992,000	5,991,000
Gross unrealized losses on investments in continuous unrealized loss position for less than 12 months	(49,000)			(174,000)				(49,000)	(174,000)		(43,000)	(161,000)	(2,000)	(7,000)	(4,000)	(6,000)
Fair value of investments in continuous unrealized loss position for 12 months or greater				24,024,000					24,024,000			4,124,000								19,900,000
Gross unrealized losses on investments in continuous unrealized loss position for 12 months or greater				(2,377,000)					(2,377,000)			(4,000)								(2,373,000)
Fair value of investments in continuous unrealized loss position	47,299,000			72,953,000				47,299,000	72,953,000		37,036,000	38,221,000	6,271,000	8,841,000	3,992,000	5,991,000				19,900,000
Gross unrealized losses on investments in continuous unrealized loss position	$ (49,000)			$ (2,551,000)				$ (49,000)	$ (2,551,000)		$ (43,000)	$ (165,000)	$ (2,000)	$ (7,000)	$ (4,000)	$ (6,000)				$ (2,373,000)