ACQUISITION - Pro Forma Information (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Business Acquisition [Line Items]
Loss on extinguishment of debt			$ 1,445	$ 0
Pro forma adjustment, inventory valuation	$ 204,943	$ 184,646	597,855	514,528
Incremental interest expense	5,032	1,991	15,812	5,521
Amortization of intangible assets	2,200	800	6,300	1,600
Tax impact of nonrecurring adjustments	$ 7,164	$ 8,158	17,452	21,104
Muuto Acquisition
Business Acquisition [Line Items]
Pro forma sales			951,795	863,725
Pro forma net earnings attributable to Knoll, Inc. stockholders			53,693	47,282
Incremental interest expense			900	2,900
Amortization of intangible assets			100	5,000
Tax impact of nonrecurring adjustments			1,100	3,800
Muuto Acquisition \| Acquisition-related Costs
Business Acquisition [Line Items]
Compensation for post combination service			2,300	2,600
Loss on extinguishment of debt			1,400	1,400
Pro forma adjustment, acquisition costs			1,600
Muuto Acquisition \| Fair Value Adjustment to Inventory
Business Acquisition [Line Items]
Pro forma adjustment, inventory valuation			$ 900	$ 900

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.

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The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

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Costs incurred to effect a business combination which have been expensed during the period. Such costs could include business integration costs, systems integration and conversion costs, and severance and other employee-related costs.

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