Retirement and Postretirement Benefits Other Than Pension Plans - Net Periodic Pension and Postretirement Benefit (Income) Expense (Detail) (Other Postretirement Benefit Plan [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Postretirement Benefit Plan [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Service cost			$ 42
Interest cost	221	194	790
Amortization of net actuarial loss	445	477	491
Amortization of prior service credit	(1,314)	(1,331)	(682)
Curtailment gain			(310)
Net postretirement benefit (income) expense	$ (648)	$ (660)	$ 331