Common and Preferred Stock - Additional Information (Detail)						1 Months Ended		12 Months Ended
	Oct. 23, 2017 shares	Oct. 10, 2017 USD ($) $ / shares shares	Mar. 01, 2017 shares	Nov. 16, 2015 shares	Nov. 12, 2015	Nov. 30, 2017 USD ($) $ / shares shares	Nov. 30, 2015 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 13, 2017 $ / shares shares	Sep. 30, 2016 shares	May 31, 2016 $ / shares	Nov. 09, 2015 shares	Oct. 23, 2015 shares
Class of Stock [Line Items]
Common stock, shares authorized			140,000,000					280,000,000	140,000,000			280,000,000	700,000,000
Common stock, par value | $ / shares								$ 0.01	$ 0.01			$ 0.01
Undesignated preferred stock, shares authorized								10,000,000	10,000,000			10,000,000
Undesignated preferred stock, par value | $ / shares								$ 0.01	$ 0.01			$ 0.01
Common stock, shares issued								119,053,414	95,680,831
Common stock, shares outstanding								119,053,414	95,680,831
Undesignated preferred stock, shares outstanding								0	0
Amount of net proceeds from issuance of securities | $								$ 564,000		$ 28,392,000
Stock split of common stock			0.2
Reverse stock split, description								The Company’s board of directors approved a 1-for-5 reverse stock split of its outstanding common stock. On March 1, 2017, the Company filed with the Secretary of State of the State of Delaware a Certificate of Amendment of the Company’s Amended and Restated Certificate of Incorporation (the “Charter Amendment”) to effect the 1-for-5 reverse stock split of the Company’s outstanding common stock (the “Reverse Stock Split”) and to reduce the authorized number of shares of the Company’s common stock from 700,000,000 to 140,000,000 shares. The Company’s common stock began trading on the NASDAQ Global Market on a split-adjusted basis when the market opened on March 3, 2017. As a result, all common stock share amounts included in these consolidated financial statements have been retroactively reduced by a factor of five, and all common stock per share amounts have been increased by a factor of five, with the exception of the Company’s common stock par value.
Common stock shares sold aggregate price | $								$ 564,000		27,843,000
Issuance costs associated with the Tel Aviv Stock Exchange | $								$ 17,000		548,000
Percentage of shares of common stock issued for offering				1.15%
Restricted warrants issued to purchase number of common stock for offering				32,000				32,000
Exercise price of restricted warrants per share in connection with the offering | $ / shares								$ 13.05						$ 7.50
Warrants exercisable period								5 years
Common stock loaned under share lending agreement, shares											1,800,000
Principal amount | $								$ 157,800,000
Nominal lending fee | $ / shares											$ 0.05
Returned shares of common stock under share lending agreement																1,800,000
Proceeds from exercise warrants | $									$ 0	10,123,000
Warrants expiration, date								Feb. 08, 2016
Senior convertible notes due August 15, 2015
Class of Stock [Line Items]
Principal amount | $											$ 100,000,000
TEL AVIV STOCK EXCHANGE
Class of Stock [Line Items]
Common stock, shares authorized															10,000,000
Number of shares sold during the period							2,770,487
Percentage of shares price per share					97.00%
Common stock shares sold aggregate price | $							$ 34,710,000			34,710,000
Share price | $ / shares							$ 13.05
Issuance costs associated with the Tel Aviv Stock Exchange | $							$ 1,432,000			$ 1,432,000
Common Stock
Class of Stock [Line Items]
Number of shares sold during the period	973,236							173,000		1,788,000
Stock split of common stock			0.2
Reverse stock split, description								On March 1, 2017, the Company effected a 1-for-5 reverse stock split of the Company’s outstanding common stock. As a result, prior to March 1, 2017, all common stock share amounts included in these consolidated financial statements have been retroactively reduced by a factor of five, and all common stock per share amounts have been increased by a factor of five, with the exception of the Company’s common stock par value.
Common stock shares sold aggregate price | $								$ 2,000		$ 18,000
Common stock loaned under share lending agreement, shares										1,800,000
Common Stock | TEL AVIV STOCK EXCHANGE
Class of Stock [Line Items]
Number of shares sold during the period										2,771,000
Common stock shares sold aggregate price | $										$ 28,000
Purchase Agreements
Class of Stock [Line Items]
Stock purchase agreements date		Oct. 10, 2017
Placement agent fees		5.00%
Purchase Agreements | Common Stock
Class of Stock [Line Items]
Number of shares sold during the period		10,166,600
Exchange price per share | $ / shares		$ 6.00
Amount of net proceeds from issuance of securities | $		$ 57,700,000
Charitable Foundation | Common Stock | Chairman of Board of Directors
Class of Stock [Line Items]
Number of shares sold during the period		166,600
At Market Issuance Sales Agreement | FBR Capital Markets & Co. | Common Stock
Class of Stock [Line Items]
Number of shares sold during the period						173,327
Exchange price per share | $ / shares						$ 3.15
Amount of net proceeds from issuance of securities | $						$ 500,000
Minimum
Class of Stock [Line Items]
Common stock, shares authorized												140,000,000
Maximum
Class of Stock [Line Items]
Common stock, shares authorized												280,000,000