Acquisitions - Summary of Preliminary Valuation of Identifiable Assets Acquired and Liabilities Assumed Upon Acquisition (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Apr. 10, 2018	Feb. 28, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 296.5			$ 188.2
Koppers Utility and Industrial Products Inc. [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		$ 1.5
Accounts receivable		20.2
Inventory		43.1
Other current assets		1.2
Property, plant and equipment		19.8
Intangible assets, net		59.4
Goodwill		76.9
Other non-current assets		0.2
Total assets acquired		222.3
Accounts payable & accrued expenses		18.7
Other non-current liabilities		0.8
Net assets acquired		$ 202.8
M.A. Energy Resources, LLC [Member]
Business Acquisition [Line Items]
Cash and cash equivalents			$ 3.2
Accounts receivable			4.9
Inventory			0.0
Other current assets			0.3
Property, plant and equipment			6.6
Intangible assets, net			19.3
Goodwill			34.0
Other non-current assets			0.0
Total assets acquired			68.3
Accounts payable & accrued expenses			2.3
Other non-current liabilities			0.0
Net assets acquired			$ 66.0