Property, Plant and Equipment - Property, Plant and Equipment (Parenthetical) (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Assets of discontinued operations held for sale	$ 67.5	$ 76.4
Koppers (Jiangsu) Carbon Chemical Company Limited [Member]
Property, Plant and Equipment [Line Items]
Assets of discontinued operations held for sale	$ 54.8	$ 56.6

Amount classified as assets attributable to disposal group held for sale or disposed of.

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