REVENUE - Net Sales by Product Category (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue from External Customer [Line Items]
Total Net Sales	$ 323,351	$ 268,694	$ 619,910	$ 525,514
Office Segment
Revenue from External Customer [Line Items]
Total Net Sales	190,747	162,599	372,365	320,597
Lifestyle Segment
Revenue from External Customer [Line Items]
Total Net Sales	132,604	106,095	247,545	204,917
Office Systems | Office Segment
Revenue from External Customer [Line Items]
Total Net Sales	104,124	94,479	207,478	194,707
Seating | Office Segment
Revenue from External Customer [Line Items]
Total Net Sales	33,471	27,193	63,428	52,855
Files and Storage | Office Segment
Revenue from External Customer [Line Items]
Total Net Sales	21,214	20,598	43,292	39,133
Ancillary | Office Segment
Revenue from External Customer [Line Items]
Total Net Sales	20,761	11,593	39,906	19,664
Other | Office Segment
Revenue from External Customer [Line Items]
Total Net Sales	11,177	8,736	18,261	14,238
Studio | Lifestyle Segment
Revenue from External Customer [Line Items]
Total Net Sales	104,807	78,482	192,374	149,389
Coverings | Lifestyle Segment
Revenue from External Customer [Line Items]
Total Net Sales	$ 27,797	$ 27,613	$ 55,171	$ 55,528