REVENUE - Net Sales by Product Category (Details) - USD ($) $ 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
Revenue from External Customer [Line Items]
Net sales	$ 371.5	$ 356.5	$ 367.3	$ 332.8	$ 354.6	$ 327.7	$ 323.4	$ 296.6	$ 1,428.1	$ 1,302.3	$ 1,132.9
Office Segment
Revenue from External Customer [Line Items]
Net sales									873.8	797.1	733.3
Office Segment | Office Systems
Revenue from External Customer [Line Items]
Net sales									449.4	439.2	422.7
Office Segment | Seating
Revenue from External Customer [Line Items]
Net sales									131.2	131.0	118.4
Office Segment | Files and Storage
Revenue from External Customer [Line Items]
Net sales									104.9	92.0	90.4
Office Segment | Ancillary
Revenue from External Customer [Line Items]
Net sales									134.4	91.9	59.5
Office Segment | Other
Revenue from External Customer [Line Items]
Net sales									53.9	43.0	42.3
Lifestyle Segment
Revenue from External Customer [Line Items]
Net sales									554.3	505.2	399.6
Lifestyle Segment | Studio
Revenue from External Customer [Line Items]
Net sales									439.9	395.2	290.6
Lifestyle Segment | Coverings
Revenue from External Customer [Line Items]
Net sales									$ 114.4	$ 110.0	$ 109.0