GOODWILL AND INTANGIBLES ASSETS, NET - Carrying Amount of Goodwill by Reportable Segment (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Goodwill [Roll Forward]
Balance at beginning of period	$ 142,113
Foreign currency translation adjustment	(9,570)
Balance at end of period	326,356
Muuto Acquisition
Goodwill [Roll Forward]
Goodwill acquired	193,813
Balance at end of period	193,462
Office Segment
Goodwill [Roll Forward]
Balance at beginning of period	36,220
Foreign currency translation adjustment	(335)
Goodwill acquired	0
Balance at end of period	35,885
Lifestyle Segment
Goodwill [Roll Forward]
Balance at beginning of period	105,893
Foreign currency translation adjustment	(9,235)
Balance at end of period	290,471
Lifestyle Segment | Muuto Acquisition
Goodwill [Roll Forward]
Goodwill acquired	$ 193,813