EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net earnings attributable to Knoll, Inc. stockholders									$ 67.5	$ 73.3	$ 80.2
Denominator:
Denominator for basic earnings per share - weighted-average shares (in shares)									48,846,000	48,657,000	48,423,000
Potentially dilutive shares resulting from stock plans (in shares)									611,000	561,000	737,000
Denominator for diluted earnings per share - weighted-average shares (in shares)									49,457,000	49,218,000	49,160,000
Antidilutive equity awards not included in weighted-average common shares—diluted (in shares)									0	20,000	0
Net earnings per share:
Basic (in dollars per share)	$ 0.21	$ 0.36	$ 0.44	$ 0.37	$ 0.50	$ 0.42	$ 0.27	$ 0.31	$ 1.38	$ 1.51	$ 1.66
Diluted (in dollars per share)	$ 0.21	$ 0.35	$ 0.44	$ 0.37	$ 0.50	$ 0.41	$ 0.27	$ 0.31	$ 1.36	$ 1.49	$ 1.63