ACQUISITION - Pro Forma Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Business Acquisition [Line Items]
Loss on extinguishment of debt			$ 1,445	$ 0
Pro forma adjustment, inventory valuation	$ 204,064	$ 168,736	392,912	329,882
Pro forman adjustment Interest expense	5,252	1,859	10,780	3,530
Amortization of intangible assets	2,200	800	4,100	1,600
Tax impact of nonrecurring adjustments	$ 4,621	$ 7,182	10,288	12,946
Muuto Acquisition
Business Acquisition [Line Items]
Pro forma sales			624,058	557,494
Pro forma net earnings attributable to Knoll, Inc. stockholders			32,591	25,601
Pro forman adjustment Interest expense			900	4,500
Amortization of intangible assets			100	3,300
Tax impact of nonrecurring adjustments			900	3,400
Acquisition-related Costs | Muuto Acquisition
Business Acquisition [Line Items]
Compensation for post combination service			1,500	1,800
Loss on extinguishment of debt			(1,400)	(1,500)
Pro forma adjustment, acquisition costs			1,500
Fair Value Adjustment to Inventory | Muuto Acquisition
Business Acquisition [Line Items]
Pro forma adjustment, inventory valuation			$ 900	$ 900