Employee Benefit Plans - Changes in Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined benefit pension plans [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net actuarial gain (loss)	$ (426)	$ (10,360)	$ 2,479
Amortization of unrecognized net actuarial loss	1,353	900	1,326
Total	927	(9,460)	3,805
OPEB Plan [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net actuarial gain (loss)	282	949	(839)
Plan amendment			3,646
Actuarial gain	(282)
Prior service credit	(698)	(800)	(179)
Total	$ (515)	$ 149	$ 2,628