Restructuring and Integration - Summary of Restructuring and Integration Charges (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Restructuring Cost and Reserve [Line Items]
Total	$ 140	$ 2,538
Charges	100	2,500
Wheeling Plant [Member]
Restructuring Cost and Reserve [Line Items]
Severance and other employee costs	140	1,576
Site closure and other associated costs		926
Integration and other associated costs		36
Total	140	$ 2,538
Reserve beginning balance	4,645
Charges	140
Paid in	(1,809)
Reserve ending balance	2,976
Wheeling Plant [Member] | Severance and Other Employee Costs [Member]
Restructuring Cost and Reserve [Line Items]
Reserve beginning balance	3,019
Charges	140
Paid in	(1,360)
Reserve ending balance	1,799
Wheeling Plant [Member] | Site Closure and Other Associated Costs [Member]
Restructuring Cost and Reserve [Line Items]
Reserve beginning balance	1,626
Paid in	(449)
Reserve ending balance	$ 1,177