Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair value of financial assets and liabilities
Fair Value Measurements	$ 27,539	$ 17,482
Convertible preferred stock warrant liability	235	117
Level 1 [Member]
Fair value of financial assets and liabilities
Fair Value Measurements	10,455	6,024
Level 2 [Member]
Fair value of financial assets and liabilities
Fair Value Measurements	17,084	11,458
Level 3 [Member]
Fair value of financial assets and liabilities
Convertible preferred stock warrant liability	235	117
Money market funds [Member]
Fair value of financial assets and liabilities
Fair Value Measurements	9,949	2,973
Money market funds [Member] | Level 1 [Member]
Fair value of financial assets and liabilities
Fair Value Measurements	9,949	2,973
U.S. treasury notes [Member]
Fair value of financial assets and liabilities
Fair Value Measurements	506	3,051
U.S. treasury notes [Member] | Level 1 [Member]
Fair value of financial assets and liabilities
Fair Value Measurements	506	3,051
Commercial paper [Member]
Fair value of financial assets and liabilities
Fair Value Measurements	4,248
Commercial paper [Member] | Level 2 [Member]
Fair value of financial assets and liabilities
Fair Value Measurements	4,248
Corporate securities [Member]
Fair value of financial assets and liabilities
Fair Value Measurements	4,517	6,022
Corporate securities [Member] | Level 2 [Member]
Fair value of financial assets and liabilities
Fair Value Measurements	4,517	6,022
U.S. government-backed securities [Member]
Fair value of financial assets and liabilities
Fair Value Measurements	8,319	5,436
U.S. government-backed securities [Member] | Level 2 [Member]
Fair value of financial assets and liabilities
Fair Value Measurements	$ 8,319	$ 5,436