Pension and Other Postretirement Employee Benefit Plans - Additional Information (Detail 1) - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	$ 0
Non-operating pension and other postretirment benefit (costs) income	1,314		$ 1,279
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	1,239		1,419
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contribution	100
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	400
Non Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	444		11
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	1,445		1,750
Employer contribution	$ 0		$ 0
Cost of Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, before Tax						$ (200)
Non-operating expense [Member]
Defined Benefit Plan Disclosure [Line Items]
Non-operating pension and other postretirment benefit (costs) income						1,300
Selling, General and Administrative Expenses [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, before Tax						$ (200)