Short -Term Investments (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Investments in continuous unrealized loss position for less than twelve months
Fair Value	$ 19,265	$ 32,904
Unrealized Losses	(13)	(60)
Commercial Paper [Member]
Investments in continuous unrealized loss position for less than twelve months
Fair Value	2,494	15,678
Unrealized Losses	(2)	(12)
Corporate bonds and notes [Member]
Investments in continuous unrealized loss position for less than twelve months
Fair Value	16,771	17,226
Unrealized Losses	$ (11)	$ (48)