Employee Benefit Plans - Components of Net Periodic Defined Benefit Cost (Income) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 372	$ 506	$ 744	$ 1,012
Expected return on plan assets	(590)	(689)	(1,180)	(1,378)
Recognized actuarial (gains) losses	365	394	730	788
Total	147	211	294	422
OPEB
Defined Benefit Plan Disclosure [Line Items]
Interest cost	15	20	30	40
Recognized actuarial (gains) losses	(63)	(54)	(126)	(108)
Total	$ (48)	$ (34)	$ (96)	$ (68)