Cash and Cash Equivalents and Investments (Details 1) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Cash and Cash Equivalents
Fair Value
Amortized Cost	$ 19,581	$ 30,423
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	19,581	30,423
Certificates of Deposit
Fair Value
Amortized Cost	551	551
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	551	551
U.S. Treasury Securities
Fair Value
Amortized Cost	131,040	192,211
Gross Unrealized Gains	38	24
Gross Unrealized Losses	0	(1)
Estimated Fair Value	131,078	192,234
Total Short-term Investments
Fair Value
Amortized Cost	131,591	192,762
Gross Unrealized Gains	38	24
Gross Unrealized Losses	0	(1)
Estimated Fair Value	131,629	192,785
Total Cash and Cash Equivalents and Investments
Fair Value
Amortized Cost	151,172	223,185
Gross Unrealized Gains	38	24
Gross Unrealized Losses	0	(1)
Estimated Fair Value	$ 151,210	$ 223,208