Postretirement Benefits Other Than Pensions - Change in Benefit Obligation of Postretirement Plans to Accrued Postretirement Liability (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Amendments		$ (2,800)
Other Postretirement Benefit Plan [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Benefit obligation at beginning of year	5,848	8,747
Interest cost	221	194	790
Actuarial loss	939	319
Amendments		(2,828)
Benefits paid	(316)	(584)
Benefit obligation at end of year	$ 6,692	$ 5,848	$ 8,747