REVENUE - Net Sales by Product Category (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue from External Customer [Line Items]
Total Sales	$ 327,737	$ 291,256	$ 947,647	$ 816,770
Office Segment
Revenue from External Customer [Line Items]
Total Sales	197,682	192,308	570,047	512,905
Lifestyle Segment
Revenue from External Customer [Line Items]
Total Sales	130,055	98,948	377,600	303,865
Office Systems | Office Segment
Revenue from External Customer [Line Items]
Total Sales	111,239	109,195	318,717	303,902
Seating | Office Segment
Revenue from External Customer [Line Items]
Total Sales	29,694	33,056	93,122	85,911
Files and Storage | Office Segment
Revenue from External Customer [Line Items]
Total Sales	23,940	22,956	67,232	62,089
Ancillary | Office Segment
Revenue from External Customer [Line Items]
Total Sales	24,631	18,380	64,537	38,044
Other | Office Segment
Revenue from External Customer [Line Items]
Total Sales	8,178	8,721	26,439	22,959
Studio | Lifestyle Segment
Revenue from External Customer [Line Items]
Total Sales	101,374	72,046	293,748	221,435
Coverings | Lifestyle Segment
Revenue from External Customer [Line Items]
Total Sales	$ 28,681	$ 26,902	$ 83,852	$ 82,430