Warrants and Derivative Liabilities - Schedule of Assumptions Used to Calculate the Fair Value of Exchanged Warrants (Detail) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015
Exchanged Warrants
Derivative [Line Items]
Risk-free interest rate	0.48%	0.66%	0.96%	0.64%	0.74%	0.73%
Expected annual dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Expected volatility	76.30%	76.76%	76.68%	73.39%	71.61%	70.42%
Term	1 year 3 months 4 days	1 year 6 months 4 days	1 year 9 months 4 days	2 years 4 days	2 years 3 months 4 days	2 years 6 months 4 days
Fair value	$ 0.4	$ 0.4	$ 0.3	$ 0.1	$ 0.2	$ 0.3
Hercules Warrants
Derivative [Line Items]
Risk-free interest rate	0.86%	1.08%	1.65%	1.31%	1.63%	1.41%
Expected annual dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Expected volatility	68.34%	70.25%	73.57%	75.32%	72.57%	74.60%
Term	4 years	4 years 3 months	4 years 6 months	4 years 9 months	5 years	5 years 3 months
Fair value	$ 0.3	$ 0.2	$ 0.2	$ 0.1	$ 0.2	$ 0.2
November 2014 Warrant
Derivative [Line Items]
Risk-free interest rate	0.77%	0.98%	1.51%	1.17%	1.44%	1.28%
Expected annual dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Expected volatility	70.01%	69.88%	70.02%	73.02%	74.18%	75.96%
Term	3 years 4 months 13 days	3 years 7 months 13 days	3 years 10 months 13 days	4 years 1 month 13 days	4 years 4 months 13 days	4 years 7 months 13 days
Fair value	$ 3.2	$ 2.6	$ 2.1	$ 1.3	$ 1.8	$ 2.5