Weighted-Average Assumptions (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Class of Warrant or Right [Line Items]
Risk-free interest rate	0.10%	[1]	0.10%	[1]
Expected dividend yield	0.00%	[2]	0.00%	[2]
Shares underlying warrants classified as liabilities	633,740		3,093,676
Market value of stock at beginning of the period	$ 20.37		$ 25.80
Market value of stock at end of period	$ 24.03		$ 36.95
Minimum
Class of Warrant or Right [Line Items]
Expected lives	3 months 18 days	[3]	9 months 18 days	[3]
Expected volatility	101.81%	[4]	99.40%	[4]
Maximum
Class of Warrant or Right [Line Items]
Expected lives	4 months 24 days	[3]	1 year 4 months 24 days	[3]
Expected volatility	121.88%	[4]	110.70%	[4]

[1] The risk-free interest rate is estimated using an average of U.S. Treasury bill interest rates that correlate to the prevailing interest rates at the valuation date.

[2] The expected dividend yield is zero as the Company has not paid any dividends to date and does not expect to pay dividends prior to the expiration of the warrants.

[3] The expected lives are based on the remaining contractual lives of the related warrants at the valuation date.

[4] The expected volatility is estimated using a blend of calculated volatility of the Company's common stock over a historical period and implied volatility in exchange-traded options associated with the Company's common stock.