Pension and Postretirement Benefits Other Than Pension Plans - Net Periodic Pension and Postretirement Benefit (Income) Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 891	$ 966	$ 2,706	$ 2,896
Expected return on plan assets	(1,051)	(1,021)	(3,098)	(3,061)
Amortization of net actuarial loss	282	307	861	921
Net settlement loss	1,218	1,567	1,218	1,567
Total net periodic pension/postretirement benefit (income) expense	1,340	1,819	1,687	2,323
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	27	30	81	90
Amortization of net actuarial loss	61	60	184	181
Amortization of prior service credit	(328)	(328)	(985)	(985)
Total net periodic pension/postretirement benefit (income) expense	$ (240)	$ (238)	$ (720)	$ (714)