Retirement and Postretirement Benefits Other Than Pension Plans - Net Periodic Pension and Postretirement Benefit (Income) Expense (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Pension Plan [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Interest cost	$ 986	$ 1,044
Expected return on plan assets	(1,188)	(1,409)
Amortization of net actuarial loss	308	148
Total net periodic pension (income) expense	106	(217)
Other Postretirement Benefit Plan [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Interest cost	53	55
Amortization of net actuarial loss	118	105
Amortization of prior service credit	(328)	(329)
Total net postretirement benefit income	$ (157)	$ (169)