Employee Benefit Plans - Components of Net Periodic Benefit Cost (Gain) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Curtailment/termination gain	$ (352)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Interest cost	94	$ 121
Expected return on plan assets	(153)	(167)
Recognized net loss due to settlement	422	195
Recognized net actuarial loss	51	81
Net periodic benefit cost (gain)	414	230
PRB Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	3	22
Interest cost	5	21
Amortization of net obligation at transition	(26)	(11)
Curtailment/termination gain	(352)
Net periodic benefit cost (gain)	$ (370)	$ 32