Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Rent expense	$ 0.6	$ 0.8	$ 2.2	$ 2.5
Letter of Credit
Long-term Purchase Commitment [Line Items]
Outstanding amount of letters of credit	1.4		1.4		$ 1.4
Gas, Gypsum, Paper, and Other Raw Materials
Long-term Purchase Commitment [Line Items]
Non capital purchased under commitments	$ 22.7	$ 22.0	$ 69.5	$ 64.8