SEGMENT INFORMATION - Segment Information Applicable to Reportable Operating Segments (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information
Number of reportable segments | segment	3
Number of operating segments | segment	3
Net sales:
Net sales	$ 711,850	$ 632,921
Operating income (loss):
Operating income (loss)	20,650	17,409
Interest income, net	92	198
Income before taxes	20,742	17,607
Depreciation and amortization:
Depreciation and amortization	3,147	3,709
Total assets:
Total assets	913,192	828,449	$ 937,335
Goodwill	73,602		$ 73,602
Business Solutions
Net sales:
Net sales	278,785	252,932
Enterprise Solutions
Net sales:
Net sales	333,418	275,635
Public Sector Solutions
Net sales:
Net sales	99,647	104,354
Operating Segments | Business Solutions
Net sales:
Net sales	278,785	252,932
Operating income (loss):
Operating income (loss)	11,301	8,765
Depreciation and amortization:
Depreciation and amortization	159	150
Total assets:
Total assets	319,909	280,889
Operating Segments | Enterprise Solutions
Net sales:
Net sales	333,418	275,635
Operating income (loss):
Operating income (loss)	16,722	15,473
Depreciation and amortization:
Depreciation and amortization	681	639
Total assets:
Total assets	536,672	484,497
Operating Segments | Public Sector Solutions
Net sales:
Net sales	99,647	104,354
Operating income (loss):
Operating income (loss)	(3,322)	(3,066)
Depreciation and amortization:
Depreciation and amortization	15	21
Total assets:
Total assets	52,285	55,536
Headquarters/Other
Operating income (loss):
Operating income (loss)	(4,051)	(3,763)
Depreciation and amortization:
Depreciation and amortization	2,292	2,899
Total assets:
Assets net of intercompany balance eliminations	4,326	7,527
Intersegment Elimination
Total assets:
Total assets	$ (7,024)	$ (11,201)