SEGMENT AND RELATED DISCLOSURES - Segment Information Applicable to Reportable Operating Segments (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information
Number of reportable segments | segment			3
Number of operating segments | segment			3
Net sales:
Net sales	$ 749,792	$ 676,165	$ 1,420,386	$ 1,248,559
Operating income (loss):
Operating income (loss)	22,440	21,010	33,892	36,174
Interest income (expense)	9	(12)	28	(26)
Income before taxes	22,449	20,998	33,920	36,148
Depreciation and amortization:
Depreciation and amortization	2,855	2,388	5,710	4,803
Total assets:
Total assets	713,329		713,329		$ 686,134
Operating Segments | Small and Medium Sized Businesses (SMB)
Net sales:
Net sales	296,420	270,962	570,053	532,208
Operating income (loss):
Operating income (loss)	11,280	10,794	19,887	22,115
Depreciation and amortization:
Depreciation and amortization	149	59	303	68
Total assets:
Total assets	243,312		243,312
Operating Segments | Large Account
Net sales:
Net sales	302,077	269,482	554,995	469,591
Operating income (loss):
Operating income (loss)	14,331	12,166	23,388	19,356
Depreciation and amortization:
Depreciation and amortization	548	366	1,142	672
Total assets:
Total assets	377,992		377,992
Operating Segments | Public Sector
Net sales:
Net sales	151,295	135,721	295,338	246,760
Operating income (loss):
Operating income (loss)	(11)	1,112	(2,624)	962
Depreciation and amortization:
Depreciation and amortization	42	41	81	81
Total assets:
Total assets	70,938		70,938
Headquarters/Other
Operating income (loss):
Operating income (loss)	(3,160)	(3,062)	(6,759)	(6,259)
Depreciation and amortization:
Depreciation and amortization	2,116	$ 1,922	4,184	$ 3,982
Total assets:
Assets net of intercompany balance eliminations	21,087		21,087
Intersegment Elimination
Total assets:
Total assets	$ (25,756)		$ (25,756)