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Pension and Employee Benefit Plans - Components of Pension and Employee Benefit Expenses (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Pension Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 59 $ 50 $ 117 $ 100
Interest cost 352 357 706 715
Expected return on plan assets (503) (488) (1,005) (976)
Amortization of net actuarial loss 64 44 129 88
Net periodic benefit cost (gain) (28) (37) (53) (73)
Health Care Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Interest cost 2 1 3 3
Amortization of loss (gain) 1 $ (1) 3 $ (3)
Net periodic benefit cost (gain) $ 3   $ 6