REVENUE - Net Sales by Product Category (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from External Customer [Line Items]
Net sales	$ 354,625	$ 327,737	$ 323,351	$ 296,559	$ 316,122	$ 291,256	$ 268,694	$ 256,820	$ 1,302,272	$ 1,132,892	$ 1,164,292
Office Segment
Revenue from External Customer [Line Items]
Net sales									782,020	722,984	768,873
Office Segment | Office Systems
Revenue from External Customer [Line Items]
Net sales									435,135	422,681	488,751
Office Segment | Seating
Revenue from External Customer [Line Items]
Net sales									128,855	118,454	124,732
Office Segment | Files and Storage
Revenue from External Customer [Line Items]
Net sales									91,537	90,362	85,696
Office Segment | Ancillary
Revenue from External Customer [Line Items]
Net sales									92,372	59,487	38,150
Office Segment | Other
Revenue from External Customer [Line Items]
Net sales									34,121	32,000	31,544
Lifestyle Segment
Revenue from External Customer [Line Items]
Net sales									520,252	409,908	395,419
Lifestyle Segment | Studio
Revenue from External Customer [Line Items]
Net sales									410,239	300,948	285,885
Lifestyle Segment | Coverings
Revenue from External Customer [Line Items]
Net sales									$ 110,013	$ 108,960	$ 109,534