Quarterly Information (Unaudited) (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
Restructuring Cost and Reserve [Line Items]
Earnings Per Share, Basic									$ 0.04	$ 0.59	$ 0.33
Net sales	$ 1,445.7	$ 1,535.5	$ 1,680.5	$ 1,697.7	$ 1,677.1	$ 1,735.6	$ 1,746.6	$ 1,658.9	$ 6,359.4	$ 6,818.2	$ 6,080.5
Operating profit	10.4	51.1	106.6	41.2	86.5	114.8	99.5	63.6	209.3	364.4	260.2
Net income (loss) attributable to AK Holding	$ (53.9)	$ 2.8	$ 66.8	$ (4.5)	33.5	$ 67.2	$ 56.6	$ 28.7	11.2	186.0	103.5
Basic and diluted earnings (loss) per share (in dollars per share)	$ (0.17)	$ 0.01	$ 0.21	$ (0.01)		$ 0.21	$ 0.18	$ 0.09
Pension Plans, Defined Benefit [Member]
Restructuring Cost and Reserve [Line Items]
Gain (Loss) Due to Pension Settlement	$ (26.9)				$ (14.5)				(26.9)	$ (14.5)	$ 0.0
Ashland Works [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Charges				$ 77.4					$ 69.3
Restructuring Credits	$ (8.1)