Warrants and Derivative Liabilities - Schedule of Assumptions Used to Calculate the Fair Value of Warrants (Detail) - USD ($) $ in Millions		3 Months Ended
	Dec. 19, 2014	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014
Exchanged warrants
Derivative [Line Items]
Risk-free interest rate		0.91%	0.56%	0.59%	0.48%	0.66%	0.96%	0.64%	0.74%	0.73%	1.00%	1.07%	0.98%	1.11%
Expected annual dividend yield		0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Expected volatility		44.12%	58.04%	70.50%	76.30%	76.76%	76.68%	73.39%	71.61%	70.42%	72.38%	76.20%	83.50%	80.99%
Term (years)		6 months 4 days	9 months 4 days	1 year 4 days	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	2 years 9 months 4 days	3 years 4 days	3 years 3 months 4 days	3 years 6 months 4 days
Fair value		$ 0.0	$ 0.1	$ 0.2	$ 0.4	$ 0.4	$ 0.3	$ 0.1	$ 0.2	$ 0.3	$ 0.5	$ 1.5	$ 2.3	$ 2.2
Hercules warrants
Derivative [Line Items]
Risk-free interest rate	1.74%	1.55%	1.57%	0.97%	0.86%	1.08%	1.65%	1.31%	1.63%	1.41%	1.73%
Expected annual dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Expected volatility	70.26%	66.51%	67.28%	67.98%	68.34%	70.25%	73.57%	75.32%	72.57%	74.60%	77.43%
Term (years)	5 years 6 months 11 days	3 years 3 months	3 years 6 months	3 years 9 months	4 years	4 years 3 months	4 years 6 months	4 years 9 months	5 years	5 years 3 months	5 years 6 months
Fair value	$ 0.2	$ 0.2	$ 0.2	$ 0.2	$ 0.3	$ 0.2	$ 0.2	$ 0.1	$ 0.2	$ 0.2	$ 0.2
November 2014 warrant
Derivative [Line Items]
Risk-free interest rate	1.64%	1.41%	1.43%	0.93%	0.77%	0.98%	1.51%	1.17%	1.44%	1.28%	1.61%
Expected annual dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Expected volatility	72.86%	66.53%	69.31%	68.96%	70.01%	69.88%	70.02%	73.02%	74.18%	75.96%	78.00%
Term (years)	5 years	2 years 7 months 13 days	2 years 10 months 13 days	3 years 1 month 13 days	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	4 years 10 months 13 days
Fair value	$ 4.3	$ 1.8	$ 2.3	$ 2.3	$ 3.2	$ 2.6	$ 2.1	$ 1.3	$ 1.8	$ 2.5	$ 3.2