REVENUE - Net Sales by Product Category (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue from External Customer [Line Items]
Total Sales	$ 296,559	$ 256,820
Office Segment
Revenue from External Customer [Line Items]
Total Sales	181,618	157,998
Lifestyle Segment
Revenue from External Customer [Line Items]
Total Sales	114,941	98,822
Office Systems | Office Segment
Revenue from External Customer [Line Items]
Total Sales	103,354	100,228
Seating | Office Segment
Revenue from External Customer [Line Items]
Total Sales	29,957	25,662
Files and Storage | Office Segment
Revenue from External Customer [Line Items]
Total Sales	22,078	18,535
Ancillary | Office Segment
Revenue from External Customer [Line Items]
Total Sales	19,145	8,071
Other | Office Segment
Revenue from External Customer [Line Items]
Total Sales	7,084	5,502
Studio | Lifestyle Segment
Revenue from External Customer [Line Items]
Total Sales	87,567	70,907
Coverings | Lifestyle Segment
Revenue from External Customer [Line Items]
Total Sales	$ 27,374	$ 27,915