Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Taxes [Line Items]
Provision for income taxes	$ 0	$ 0	$ 0
Deferred tax assets attributable to excess stock option benefits	5,522,000	3,314,000
Increase (decrease) in net valuation allowance of deferred tax asset	30,900,000	18,500,000
Options to purchase common stock
Income Taxes [Line Items]
Deferred tax assets attributable to excess stock option benefits	4,200,000
Internal Revenue Service (IRS)
Income Taxes [Line Items]
Net operating losses	128,000,000
Internal Revenue Service (IRS) | Minimum
Income Taxes [Line Items]
Net operating loss carryforwards, expiry date	2024
Internal Revenue Service (IRS) | Research Tax Credit Carryforward
Income Taxes [Line Items]
Research and development tax credit carryforwards	1,200,000
Internal Revenue Service (IRS) | Research Tax Credit Carryforward | Minimum
Income Taxes [Line Items]
Research and development tax credits, expiry date	2028
State and Local Jurisdiction
Income Taxes [Line Items]
Net operating losses	111,300,000
State and Local Jurisdiction | Minimum
Income Taxes [Line Items]
Net operating loss carryforwards, expiry date	2014
State and Local Jurisdiction | Research Tax Credit Carryforward
Income Taxes [Line Items]
Research and development tax credit carryforwards	$ 1,300,000