Warrants and Warrant Derivative Liability - Additional Information (Detail) $ / shares in Units, $ in Thousands		3 Months Ended	12 Months Ended
	Oct. 16, 2009 USD ($) $ / shares shares	Mar. 31, 2015 USD ($)	Dec. 31, 2017 shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 shares
Derivative [Line Items]
Common stock issued under private placement, value | $		$ 52,800
Issuance of common stock, net of transaction costs | $				$ 64,613
Warrants outstanding			0	0
Proceeds from exercise of warrants | $					$ 2,713
Gain (loss) on change in fair value of derivative liabilities | $				$ 8,170	$ (1,106)
Warrants Issued October 2009
Derivative [Line Items]
Warrants outstanding			0	0		8,087,388
Warrants exercised					1,844,585
Proceeds from exercise of warrants | $				$ 2,700
Warrants expired					6,242,803
Gain (loss) on change in fair value of derivative liabilities | $					$ 100
October 2009 Private Placement
Derivative [Line Items]
Common stock issued under private placement, value | $	$ 70,000
Issuance of common stock, net of transaction costs | $	$ 62,300
Common stock issued under private placement, shares	66,400,000
Number of shares that each ADS represents	1
Common stock issued under private placement, price per share | $ / shares	$ 1.00
October 2009 Private Placement | Warrants issued to three former officers
Derivative [Line Items]
Number of warrants issued			900,000
October 2009 Private Placement | Warrants Issued October 2009
Derivative [Line Items]
Number of warrants issued	35,200,000
Description of pricing variability feature and classification of warrants			The warrants issued in connection with the October 2009 financing contained a pricing variability feature which provided for an increase to the exercise price if the exchange rate between the U.S. dollar and British pound adjusts such that the warrants could be exercised at a price less than the £0.5 par value of the common stock—that is, if the exchange rate exceeded U.S. $3.00 per £1.0 sterling. Due to the potential variable nature of the exercise price, the warrants were not considered to be indexed to the Company’s common stock. Accordingly, the warrants did not qualify for the exception to classify the warrants within equity and were classified as a derivative liability
October 2009 Private Placement | Warrants Issued October 2009 | Maximum
Derivative [Line Items]
Warrants issued, number of shares called by the warrant	36,100,000
October 2009 Private Placement | Warrants Issued October 2009 | American Depositary Shares
Derivative [Line Items]
Warrant term	5 years
Warrants issued, number of shares called by a warrant	0.5
Exercise price of the warrants | $ / shares	$ 1.50
October 2009 Private Placement | Bridge Loan
Derivative [Line Items]
Bridge notes converted in conjunction with the private placement, amount | $	$ 3,600
Common stock issued under private placement, shares	4,000,000
Number of shares that each ADS represents	1
Bridge notes converted in conjunction with the private placement, price per share | $ / shares	$ 0.90
October 2009 Private Placement | Bridge Loan | Warrants Issued October 2009 | American Depositary Shares
Derivative [Line Items]
Warrant term	5 years
Warrants issued, number of shares called by a warrant	0.5
Exercise price of the warrants | $ / shares	$ 1.50