ACQUISITIONS - Pro Forma (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Loss on extinguishment of debt	$ 0.4	$ 1.4	$ 0.0
Acquisition related inventory valuation	879.1	820.8	718.3
Interest expense	21.7	20.9	7.4
Amortization of intangibles	8.9	8.9	3.3
Income tax impact	$ 23.4	24.9	(1.6)
Muuto Acquisition
Business Acquisition [Line Items]
Pro forma sales		1,306.4	1,203.2
Pro forma net earnings attributable to Knoll, Inc. stockholders		79.0	77.9
Interest expense		0.1	1.7
Amortization of intangibles		0.8	6.6
Income tax impact		1.3	4.6
Muuto Acquisition \| Acquisition-related Costs
Business Acquisition [Line Items]
Compensation for post combination services		3.2	3.5
Loss on extinguishment of debt		1.4	1.2
Acquisition costs			1.9
Muuto Acquisition \| Fair Value Adjustment to Inventory
Business Acquisition [Line Items]
Acquisition related inventory valuation		$ 0.9	$ 0.9

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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The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

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