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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 953	$ 986	$ 1,930	$ 1,972
Expected return on plan assets	(1,006)	(1,188)	(2,040)	(2,376)
Amortization of net actuarial loss	322	309	614	617
Total net periodic pension/postretirement benefit (income) expense	269	107	504	213
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	30	53	60	106
Amortization of net actuarial loss	62	118	121	236
Amortization of prior service credit	(329)	(329)	(657)	(657)
Total net periodic pension/postretirement benefit (income) expense	$ (237)	$ (158)	$ (476)	$ (315)