Financial Instruments Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Total cash and cash equivalents, Carrying Amount	$ 36,141	$ 50,332	$ 82,565	$ 114,094
Total, Carrying Amount	71,219	98,711
Warrant Liabilities	2,190	2,935
Total, Estimated Fair Value	71,219	98,711
Cash and Cash Equivalents
Derivatives, Fair Value [Line Items]
Total cash and cash equivalents, Carrying Amount	36,141	50,332
Total cash and cash equivalents, Estimated Fair Value	36,141	50,332
Cash and Cash Equivalents | Cash
Derivatives, Fair Value [Line Items]
Total cash and cash equivalents, Carrying Amount	18,696	14,221
Total cash and cash equivalents, Estimated Fair Value	18,696	14,221
Cash and Cash Equivalents | Money market funds
Derivatives, Fair Value [Line Items]
Total cash and cash equivalents, Carrying Amount	17,445	36,111
Total cash and cash equivalents, Estimated Fair Value	17,445	36,111
Total short-term investments | Certificates of deposit
Derivatives, Fair Value [Line Items]
Total short-term investments, Carrying Amount	32,648	45,949
Total short-term investments, Estimated Fair Value	32,648	45,949
Restricted cash | Certificates of deposit
Derivatives, Fair Value [Line Items]
Total cash and cash equivalents, Carrying Amount	2,430	2,430
Total cash and cash equivalents, Estimated Fair Value	2,430	2,430
Embedded Derivative
Derivatives, Fair Value [Line Items]
Liabilities, Carrying Amount	3,038	3,783
Liabilities, Estimated Fair Value	3,038	3,783
Embedded Derivative | Debt Redemption Features
Derivatives, Fair Value [Line Items]
Total embedded derivatives, Carrying Amount	848	848
Total embedded derivatives, Estimated Fair Value	848	848
Embedded Derivative | Warrant Liability
Derivatives, Fair Value [Line Items]
Warrant Liabilities	2,190	2,935
Liabilities, Estimated Fair Value	2,190	2,935
Level 1
Derivatives, Fair Value [Line Items]
Total, Estimated Fair Value	71,219	98,711
Level 1 | Cash and Cash Equivalents
Derivatives, Fair Value [Line Items]
Total cash and cash equivalents, Estimated Fair Value	36,141	50,332
Level 1 | Cash and Cash Equivalents | Cash
Derivatives, Fair Value [Line Items]
Total cash and cash equivalents, Estimated Fair Value	18,696	14,221
Level 1 | Cash and Cash Equivalents | Money market funds
Derivatives, Fair Value [Line Items]
Total cash and cash equivalents, Estimated Fair Value	17,445	36,111
Level 1 | Total short-term investments | Certificates of deposit
Derivatives, Fair Value [Line Items]
Total short-term investments, Estimated Fair Value	32,648	45,949
Level 1 | Restricted cash | Certificates of deposit
Derivatives, Fair Value [Line Items]
Total cash and cash equivalents, Estimated Fair Value	2,430	2,430
Level 3 | Embedded Derivative
Derivatives, Fair Value [Line Items]
Liabilities, Estimated Fair Value	3,038	3,783
Level 3 | Embedded Derivative | Debt Redemption Features
Derivatives, Fair Value [Line Items]
Total embedded derivatives, Estimated Fair Value	848	848
Level 3 | Embedded Derivative | Warrant Liability
Derivatives, Fair Value [Line Items]
Liabilities, Estimated Fair Value	$ 2,190	$ 2,935