UNAUDITED SUPPLEMENTARY QUARTERLY INFORMATION (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	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, 2017	Dec. 31, 2016	Dec. 31, 2015
UNAUDITED SUPPLEMENTARY QUARTERLY INFORMATION
Net (loss) income	$ 2,169	$ (8,072)	$ (1,712)	$ (3,018)	$ (4,491)	$ 4,208	$ 8,353	$ 12,985	$ (14,988)	$ 2,633	$ (14,116)	$ 6,021	$ (10,634)	$ 21,055	$ (20,450)
Net (loss) income per share:
Basic (in dollars per share)	$ 0.01	$ (0.03)	$ (0.01)	$ (0.01)	$ (0.01)	$ 0.01	$ 0.03	$ 0.04	$ (0.05)	$ 0.01	$ (0.05)	$ 0.02	$ (0.03)	$ 0.07	$ (0.07)
Diluted (in dollars per share)	$ 0.01	$ (0.03)	$ (0.01)	$ (0.01)	$ (0.01)	$ 0.01	$ 0.03	$ 0.04	$ (0.05)	$ 0.01	$ (0.05)	$ 0.02	$ (0.03)	$ 0.07	$ (0.07)
Weighted average common shares outstanding (thousands) (note 12):
Basic (in shares)	313,887	314,077	308,523	299,575	299,518	298,510	298,237	298,242	300,107	300,530	300,530	297,255	313,887	298,772	300,341
Diluted (in shares)	313,887	314,077	308,523	299,575	301,102	301,045	299,791	298,554	300,107	300,530	300,530	297,266	313,887	300,474	300,341