Fair Value of Financial Assets And Liabilities Measured At Fair Value And Classification By Level Of Input (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	$ 15,274	$ 17,482
Convertible preferred stock warrant liabilities	157	117	69
Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	5,923	2,973
U.S. government-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	9,351	5,436
U.S. treasury notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		3,051
Corporate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		6,022
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	5,923	6,024
Level 1 | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	5,923	2,973
Level 1 | U.S. treasury notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		3,051
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	9,351	11,458
Level 2 | U.S. government-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	9,351	5,436
Level 2 | Corporate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments		6,022
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible preferred stock warrant liabilities	$ 157	$ 117