Retirement 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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension Plan [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Interest cost	$ 986	$ 1,045	$ 1,972	$ 2,089
Expected return on plan assets	(1,188)	(1,410)	(2,376)	(2,819)
Amortization of net actuarial loss	309	147	617	295
Total net periodic pension (income) expense	107	(218)	213	(435)
Other Postretirement Benefit Plan [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Interest cost	53	54	106	109
Amortization of net actuarial loss	118	104	236	209
Amortization of prior service credit	(329)	(328)	(657)	(657)
Total net postretirement benefit income	$ (158)	$ (170)	$ (315)	$ (339)