Common and Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended			6 Months Ended
	Mar. 31, 2021	Feb. 28, 2021	Feb. 28, 2018	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Class of Stock [Line Items]
Common stock, shares authorized				400,000,000		400,000,000	400,000,000
Common stock, par value				$ 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				$ 0.01		$ 0.01	$ 0.01
Common stock, shares issued				249,617,550		242,117,089	249,617,550
Common stock, shares outstanding				249,617,550		242,117,089	249,617,550
Preferred stock, shares outstanding				0		0	0
Debt issuance amount	$ 230,000
Proceeds from market price stock purchase				$ 100			$ 106	$ 14
Market price stock purchase plan							25,000
Convertible Promissory Note | Mann Group
Class of Stock [Line Items]
Conversion of notes accrued interest to common shares, value					$ 400
Debt issuance amount					$ 9,600	$ 7,000	$ 9,600
Conversion of notes to common shares, shares					4,000,000	2,800,000	4,000,000
2024 Convertible Notes
Class of Stock [Line Items]
Debt issuance amount		$ 5,000
Conversion of notes to common shares, shares		1,666,667
Controlled Equity Offering Sales Agreement | Cantor Fitzgerald
Class of Stock [Line Items]
Stock sales agreements date			Feb. 28, 2018
Controlled Equity Offering Sales Agreement | Cantor Fitzgerald | Common Stock | Maximum
Class of Stock [Line Items]
Amount of net proceeds from issuance of securities			$ 50,000
At Market Issuance Sales Agreement | Cantor Fitzgerald | Common Stock
Class of Stock [Line Items]
Amount of net proceeds from issuance of securities							$ 1,900	$ 12,900
Number of shares sold during the period							578,063	7,871,461
At Market Issuance Sales Agreement | Cantor Fitzgerald | Common Stock | Weighted Average
Class of Stock [Line Items]
Exchange price per share				$ 3.26			$ 3.26	$ 1.64