EQUITY - Schedule of Change in Equity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 408.4	$ 392.8	$ 386.5	$ 369.1	$ 366.4	$ 358.8	$ 386.5	$ 358.8
Net earnings	17.5	21.7	18.0	20.3	13.1	15.3	57.1	48.7
Other comprehensive loss	(14.0)		(3.6)	0.5	(2.2)	(0.8)	(17.6)	(2.4)
Stock-based compensation, net of forfeitures	3.1	2.4	2.2	2.2	2.1	2.4
Cash dividend	(8.6)	(8.6)	(7.5)	(7.1)	(7.9)	(7.3)
Purchase of common stock	(0.3)		(3.0)		(2.4)	(2.0)
Other		0.1	0.2
Ending Balance	$ 406.1	$ 408.4	$ 392.8	$ 385.0	$ 369.1	$ 366.4	406.1	385.0
Cash dividends (in dollars per share)	$ 0.17	$ 0.17	$ 0.15	$ 0.15	$ 0.15	$ 0.15
Purchase of common stock, shares (in shares)	11,465		141,738			95,412
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 0.5	$ 0.5	$ 0.5	$ 0.5	$ 0.5	$ 0.5	0.5	0.5
Ending Balance	0.5	0.5	0.5	0.5	0.5	0.5	0.5	0.5
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	60.9	58.4	58.8	54.6	54.9	54.5	58.8	54.5
Stock-based compensation, net of forfeitures	3.1	2.4	2.2	2.2	2.1	2.4
Purchase of common stock	(0.3)		(3.0)		(2.4)	(2.0)
Other		0.1	0.4
Ending Balance	63.7	60.9	58.4	56.8	54.6	54.9	63.7	56.8
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	419.0	405.9	395.4	366.8	361.6	347.3	395.4	347.3
Net earnings	17.5	21.7	18.0	20.3	13.1	15.3
Cash dividend	(8.6)	(8.6)	(7.5)	(7.1)	(7.9)	(7.3)
Ending Balance	427.9	419.0	405.9	380.0	366.8	361.6	427.9	380.0
Retained Earnings | ASU 2018-02
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASU 2018-02						6.3
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(72.0)	(72.0)	(68.4)	(53.1)	(50.9)	(43.8)	(68.4)	(43.8)
Other comprehensive loss	(14.0)		(3.6)	0.5	(2.2)	(0.8)
Ending Balance	(86.0)	(72.0)	(72.0)	(52.6)	(53.1)	(50.9)	(86.0)	(52.6)
Accumulated Other Comprehensive Income (Loss) | ASU 2018-02
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASU 2018-02						(6.3)
Total Knoll, Inc. Stockholders' Equity
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	408.4	392.8	386.3	368.8	366.1	358.5	386.3	358.5
Net earnings	17.5	21.7	18.0	20.3	13.1	15.3
Other comprehensive loss	(14.0)		(3.6)	0.5	(2.2)	(0.8)
Stock-based compensation, net of forfeitures	3.1	2.4	2.2	2.2	2.1	2.4
Cash dividend	(8.6)	(8.6)	(7.5)	(7.1)	(7.9)	(7.3)
Purchase of common stock	(0.3)		(3.0)		(2.4)	(2.0)
Other		0.1	0.4
Ending Balance	406.1	408.4	392.8	384.7	368.8	366.1	406.1	384.7
Noncontrolling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	0.0	0.0	0.2	0.3	0.3	0.3	0.2	0.3
Net earnings						0.0
Other			(0.2)
Ending Balance	$ 0.0	$ 0.0	$ 0.0	$ 0.3	$ 0.3	$ 0.3	$ 0.0	$ 0.3