Pension & Postretirement Medical Benefit Plans - Defined Benefit Pension Plan (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plan Net Periodic Benefit Cost [Abstract]
Service cost – benefits earned during year	$ 560	$ 707	$ 1,114	$ 1,406
Interest cost on projected benefit obligation	1,313	1,228	2,611	2,442
Expected return on plan assets	(2,211)	(2,390)	(4,398)	(4,753)
Amortization of prior service cost	6	7	13	13
Amortization of net loss	808	758	1,607	1,508
Net periodic pension cost	$ 476	$ 310	$ 947	$ 616