QUARTERLY RESULTS (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Sales	$ 354,625	$ 327,737	$ 323,351	$ 296,559	$ 316,122	$ 291,256	$ 268,694	$ 256,820	$ 1,302,272	$ 1,132,892	$ 1,164,292
Gross profit	131,732	122,794	119,287	107,711	112,337	106,610	99,958	95,674	481,524	414,579	445,976
Net earnings	24,542	20,323	13,123	15,267	32,679	19,161	12,960	15,396	73,255	80,192	82,114
Net earnings attributable to Knoll, Inc. stockholders	$ 24,542	$ 20,323	$ 13,124	$ 15,259	$ 32,693	$ 19,132	$ 12,938	$ 15,404	$ 73,248	$ 80,163	$ 82,084
Earnings per share—Basic (in dollars per share)	$ 0.50	$ 0.42	$ 0.27	$ 0.31	$ 0.67	$ 0.39	$ 0.27	$ 0.32	$ 1.51	$ 1.66	$ 1.71
Earnings per share—Diluted (in dollars per share)	$ 0.50	$ 0.41	$ 0.27	$ 0.31	$ 0.67	$ 0.39	$ 0.26	$ 0.31	$ 1.49	$ 1.63	$ 1.68
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Pension settlement charges	$ 500	$ 600	$ 4,600		$ 2,200
Write-off of property, plant, and equipment					16,300				$ 0	$ 16,306	$ 0
Restructuring charges							$ 2,200		2,614	2,150	0
Results include impact of Tax Cuts and Jobs Act	1,700				$ 26,600				1,700
Corporate
Quarterly Financial Information Disclosure [Abstract]
Sales									0	0	0
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Acquisition costs	200	200	500	$ 1,000					1,300
Office
Quarterly Financial Information Disclosure [Abstract]
Sales									782,020	722,984	$ 768,873
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Pension settlement charges									5,700	2,200
Write-off of property, plant, and equipment										16,300
Restructuring charges	$ 100	$ 1,200	$ 800	$ 500					$ 2,600	$ 2,200