Pension and Postretirement Benefits Other Than Pension Plans - Net Periodic Pension and Postretirement Benefit (Income) Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 901	$ 953	$ 1,815	$ 1,930
Expected return on plan assets	(1,011)	(1,006)	(2,047)	(2,040)
Amortization of net actuarial loss	297	322	579	614
Total net periodic pension/postretirement benefit (income) expense	187	269	347	504
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	28	30	54	60
Amortization of net actuarial loss	66	62	123	121
Amortization of prior service credit	(329)	(329)	(657)	(657)
Total net periodic pension/postretirement benefit (income) expense	$ (235)	$ (237)	$ (480)	$ (476)