Restructuring and Integration - Summary of Restructuring and Integration Charges (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Restructuring Cost and Reserve [Line Items]
Total	$ 306	$ 4,047	$ 446	$ 6,585
Charges	300	4,000	400	6,600
Wheeling Plant [Member]
Restructuring Cost and Reserve [Line Items]
Severance and other employee costs	306	1,814	446	3,390
Site closure and other associated costs		2,011		2,938
Integration and other associated costs		222		257
Total	306	$ 4,047	446	$ 6,585
Reserve beginning balance			4,645
Charges			446
Paid in			(2,287)
Reserve ending balance	2,804		2,804
Wheeling Plant [Member] | Severance and Other Employee Costs [Member]
Restructuring Cost and Reserve [Line Items]
Reserve beginning balance			3,019
Charges			446
Paid in			(1,673)
Reserve ending balance	1,792		1,792
Wheeling Plant [Member] | Site Closure and Other Associated Costs [Member]
Restructuring Cost and Reserve [Line Items]
Reserve beginning balance			1,626
Paid in			(614)
Reserve ending balance	$ 1,012		$ 1,012