NET INCOME (LOSS) PER COMMON SHARE - Computation of Earnings Per Share (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, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic:
Net (loss) income attributable to ISG	$ (2,650)	$ 1,428	$ (302)	$ (606)	$ (8,150)	$ 732	$ 1,612	$ (699)	$ (2,130)	$ (6,505)	$ 4,841
Weighted average common shares	43,423	43,305	43,058	42,316	37,842	35,707	35,609	37,340	43,025	36,625	37,186
Basic (loss) earnings per share attributable to ISG	$ (0.06)	$ 0.03	$ (0.01)	$ (0.01)	$ (0.22)	$ 0.02	$ 0.05	$ (0.02)	$ (0.05)	$ (0.18)	$ 0.13
Diluted:
Net (loss) income attributable to ISG	$ (2,650)	$ 1,428	$ (302)	$ (606)	$ (8,150)	$ 732	$ 1,612	$ (699)	$ (2,130)	$ (6,505)	$ 4,841
Interest expense of convertible debt, net of tax											80
Net (loss) income attributable to ISG, as adjusted									$ (2,130)	$ (6,505)	$ 4,921
Basic weighted average common shares	43,423	43,305	43,058	42,316	37,842	35,707	35,609	37,340	43,025	36,625	37,186
Potential common shares											1,750
Diluted weighted average common shares	43,423	44,658	43,058	42,316	37,842	36,873	36,719	37,340	43,025	36,625	38,936
Diluted (loss) earnings per share attributable to ISG	$ (0.06)	$ 0.03	$ (0.01)	$ (0.01)	$ (0.22)	$ 0.02	$ 0.04	$ (0.02)	$ (0.05)	$ (0.18)	$ 0.13