Pension and Employee Benefit Plans - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan contributions	$ 600,000	$ 300,000
Unfunded liability plans	9,100,000		$ 9,600,000
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Anticipated contribution to fund the pension plan during the reminder of the fiscal year	200,000
Health Care Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Anticipated contribution to fund the pension plan during the reminder of the fiscal year	$ 24,000