ACQUISITIONS - Pro Forma (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Loss on extinguishment of debt	$ 1,445	$ 0	$ 0
Acquisition related inventory valuation	820,748	718,313	718,316
Interest expense	20,911	7,483	5,405
Amortization of intangibles	8,900	3,300	3,300
Income tax impact	24,896	(1,600)	$ 45,424
Muuto Acquisition
Business Acquisition [Line Items]
Pro forma sales	1,306,420	1,203,158
Pro forma net earnings attributable to Knoll, Inc. stockholders	78,964	77,892
Interest expense	100	1,700
Amortization of intangibles	800	6,600
Income tax impact	1,300	4,600
Muuto Acquisition | Acquisition-related Costs
Business Acquisition [Line Items]
Compensation for post combination services	3,200	3,500
Loss on extinguishment of debt	1,400	1,200
Acquisition costs		1,900
Muuto Acquisition | Fair Value Adjustment to Inventory
Business Acquisition [Line Items]
Acquisition related inventory valuation	$ 900	$ 900