NET INCOME PER COMMON SHARE - Computation of Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic:
Net income (loss) attributable to ISG	$ 2,362	$ (302)	$ 2,526	$ (908)
Weighted average common shares	44,655	43,058	44,180	42,687
Earnings (loss) per share attributable to ISG	$ 0.05	$ (0.01)	$ 0.06	$ (0.02)
Diluted:
Net income (loss) attributable to ISG	$ 2,362	$ (302)	$ 2,526	$ (908)
Net income (loss) attributable to ISG, as adjusted	$ 2,362	$ (302)	$ 2,526	$ (908)
Basic weighted average common shares	44,655	43,058	44,180	42,687
Potential common shares	1,431		1,793
Diluted weighted average common shares	46,086	43,058	45,973	42,687
Diluted earnings (loss) per share attributable to ISG	$ 0.05	$ (0.01)	$ 0.05	$ (0.02)