Pension & Postretirement Medical Benefit Plans - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum required pension plan contribution	$ 4,800		$ 4,800
Available credit balance	52,500		52,500
Expected voluntary contribution for the year 2019			3,000
Net periodic interest cost	1,313	$ 1,228	2,611	$ 2,442
Prior service cost	6	7	13	13
Net periodic cost post retirement medical benefit plan	476	$ 310	947	$ 616
Postemployment Retirement Benefits [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net periodic interest cost	115		228
Prior service cost	56		111
Net periodic cost post retirement medical benefit plan	$ 59		$ 117