Retirement 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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 1,005	$ 1,053	$ 2,977	$ 3,142
Expected return on plan assets	(1,142)	(1,379)	(3,518)	(4,198)
Amortization of net actuarial loss	258	180	875	475
Net settlement loss	1,593		1,593
Total net periodic pension and postretirement benefit (income) expense	1,714	(146)	1,927	(581)
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	53	56	159	165
Amortization of net actuarial loss	118	125	354	334
Amortization of prior service credit	(329)	(328)	(986)	(985)
Total net periodic pension and postretirement benefit (income) expense	$ (158)	$ (147)	$ (473)	$ (486)