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Employee Benefit plans - Components of Net Periodic Defined Benefit Cost (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Defined Benefit Pension Plans [Member]
   
Pension Plans, Postretirement and Other Employee Benefits [Line Items]    
Interest cost $ 572 $ 628
Expected return on plan assets (1,000) (914)
Recognized actuarial gain (losses) 309 331
Total (119) 45
OPEB [Member]
   
Pension Plans, Postretirement and Other Employee Benefits [Line Items]    
Interest cost 26 39
Amortization of prior service credit (171) (174)
Recognized actuarial gain (losses) (37) (25)
Total $ (182) $ (160)