EQUITY Schedule of Change in Equity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 386.5	$ 358.8
Net earnings	18.0	15.3
Other comprehensive (loss) income	(3.6)	(0.8)
Stock-based compensation, net of forfeitures	2.2	2.4
Cash dividend ($0.15 per share)	(7.5)	(7.3)
Purchase of common stock	(3.0)	(2.0)
Other	0.2
Ending Balance	$ 392.8	$ 366.4
Cash dividends (in dollars per share)	$ 0.15	$ 0.15
Purchase of common stock, shares (in shares)	141,738	95,412
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 0.5	$ 0.5
Ending Balance	0.5	0.5
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	58.8	54.5
Stock-based compensation, net of forfeitures	2.2	2.4
Purchase of common stock	(3.0)	(2.0)
Other	0.4
Ending Balance	58.4	54.9
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	395.4	347.3
Net earnings	18.0	15.3
Other comprehensive (loss) income	0.0
Cash dividend ($0.15 per share)	(7.5)	(7.3)
Ending Balance	405.9	361.5
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	(68.4)	(43.8)
Other comprehensive (loss) income	(3.6)	(0.8)
Ending Balance	(72.0)	(50.8)
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	386.3	358.5
Net earnings	18.0	15.3
Other comprehensive (loss) income	(3.6)	(0.8)
Stock-based compensation, net of forfeitures	2.2	2.4
Cash dividend ($0.15 per share)	(7.5)	(7.3)
Purchase of common stock	(3.0)	(2.0)
Other	0.4
Ending Balance	392.8	366.1
Noncontrolling Interests
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	0.2	0.3
Net earnings	0.0	0.0
Other	(0.2)
Ending Balance	$ 0.0	$ 0.3