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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 966	$ 1,005	$ 2,896	$ 2,977
Expected return on plan assets	(1,021)	(1,142)	(3,061)	(3,518)
Amortization of net actuarial loss	307	258	921	875
Net settlement loss	1,567	1,593	1,567	1,593
Total net periodic pension/postretirement benefit (income) expense	1,819	1,714	2,323	1,927
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	30	53	90	159
Amortization of net actuarial loss	60	118	181	354
Amortization of prior service credit	(328)	(329)	(985)	(986)
Total net periodic pension/postretirement benefit (income) expense	$ (238)	$ (158)	$ (714)	$ (473)