SEGMENT AND RELATED DISCLOSURES - Segment Information Applicable to Reportable Operating Segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	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, 2016	Dec. 31, 2015	Dec. 31, 2014
Net sales:
Net sales	$ 735,548	$ 708,485	$ 676,165	$ 572,394	$ 684,323	$ 680,769	$ 627,622	$ 581,259	$ 2,692,592	$ 2,573,973	$ 2,463,339
Operating income (loss):
Operating income (loss)	21,901	22,445	21,010	15,164	22,891	21,861	19,623	14,179	80,520	78,554	71,454
Interest expense	(14)	(27)	(12)	(14)	(20)	(29)	(39)	1	(67)	(87)	(86)
Income before taxes	21,887	$ 22,418	$ 20,998	$ 15,150	22,871	$ 21,832	$ 19,584	$ 14,180	80,453	78,467	71,368
Depreciation and amortization:
Depreciation and amortization									10,453	8,961	8,092
Total assets:
Total assets	686,134				639,074				686,134	639,074
Goodwill	73,602				51,276				73,602	51,276
Small and Medium Sized Businesses Segment
Total assets:
Goodwill	7,366								7,366
Large Account Segment
Total assets:
Goodwill	66,236				51,276				66,236	51,276
Operating Segments | Small and Medium Sized Businesses Segment
Net sales:
Net sales									1,091,182	1,040,586	1,037,620
Operating income (loss):
Operating income (loss)									41,596	42,855	39,608
Depreciation and amortization:
Depreciation and amortization									425	24	5
Total assets:
Total assets	240,665				207,147				240,665	207,147
Goodwill	7,366								7,366
Operating Segments | Large Account Segment
Net sales:
Net sales									1,011,990	961,013	850,796
Operating income (loss):
Operating income (loss)									42,504	41,234	39,229
Depreciation and amortization:
Depreciation and amortization									1,784	1,297	1,368
Total assets:
Total assets	361,431				320,633				361,431	320,633
Goodwill	66,236								66,236
Operating Segments | Public Sector
Net sales:
Net sales									589,420	572,374	574,923
Operating income (loss):
Operating income (loss)									8,561	6,879	3,621
Depreciation and amortization:
Depreciation and amortization									160	156	124
Total assets:
Total assets	95,278				73,374				95,278	73,374
Headquarters/Other
Operating income (loss):
Operating income (loss)									(12,141)	(12,414)	(11,004)
Depreciation and amortization:
Depreciation and amortization									8,084	7,484	$ 6,595
Total assets:
Assets net of intercompany balance eliminations	(11,240)				37,920				(11,240)	37,920
Intersegment Elimination.
Total assets:
Total assets	$ (49,937)				$ (36,752)				$ (49,937)	$ (36,752)