Investments (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Summary of Fair Value of Investments with Gross Unrealized Losses
Fair value of investments in continuous unrealized loss position for less than 12 months	$ 62,476	$ 47,299
Gross unrealized losses on investments in continuous unrealized loss position for less than 12 months	(80)	(49)
Fair value of investments in continuous unrealized loss position	62,476	47,299
Gross unrealized losses on investments in continuous unrealized loss position	(80)	(49)
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	53,936	37,036
Gross unrealized losses on investments in continuous unrealized loss position for less than 12 months	(78)	(43)
Fair value of investments in continuous unrealized loss position	53,936	37,036
Gross unrealized losses on investments in continuous unrealized loss position	(78)	(43)
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	4,542	6,271
Gross unrealized losses on investments in continuous unrealized loss position for less than 12 months	(1)	(2)
Fair value of investments in continuous unrealized loss position	4,542	6,271
Gross unrealized losses on investments in continuous unrealized loss position	(1)	(2)
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	3,998	3,992
Gross unrealized losses on investments in continuous unrealized loss position for less than 12 months	(1)	(4)
Fair value of investments in continuous unrealized loss position	3,998	3,992
Gross unrealized losses on investments in continuous unrealized loss position	$ (1)	$ (4)