Employee Retirement Plans - Reconciliations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in plan assets:
Fair value of plan assets at beginning of year	$ 33,903
Fair value of plan assets at end of year	38,053	$ 33,903
Amounts recognized in the consolidated balance sheets consisted
Retiree health benefit obligation	6,240	6,809
Pension plan
Components of net periodic cost:
Service cost	409	356	$ 321
Interest cost	1,555	1,613	1,639
Expected return on plan assets	(1,901)	(1,790)	(1,824)
Amortization of net (gain) loss	706	723	724
Net periodic pension cost	769	902	860
Employer contributions during the period	7,047	1,669
Change in projected benefit obligation:
Benefit obligation at beginning of year	43,664	43,664	39,407
Actuarial (gain) loss	(3,296)	3,571
Benefits paid	(1,391)	(1,283)
Curtailment	(759)
Benefit obligation at end of year	40,182	43,664	43,664
Change in plan assets:
Fair value of plan assets at beginning of year	33,903	29,223
Actual return on plan assets	(1,506)	4,294
Benefits paid	(1,391)	(1,283)
Fair value of plan assets at end of year	38,053	33,903	29,223
Funded status at end of year	(2,129)	(9,761)
Salaried Pension Plan
Accumulated benefit obligation	40,182	42,876
Pension plan | Minimum
Components of net periodic cost:
Employer contributions during the period	47
Other postretirement benefit cost
Components of net periodic cost:
Service cost	189	205	210
Interest cost	233	278	278
Amortization of net (gain) loss	(211)	(107)	(127)
Net periodic pension cost	211	376	361
Change in projected benefit obligation:
Benefit obligation at beginning of year	6,949	6,949	7,333
Participant contributions	25	25
Actuarial (gain) loss	(926)	(853)
Benefits paid	(50)	(39)
Benefit obligation at end of year	6,420	6,949	$ 6,949
Change in plan assets:
Benefits paid	$ (50)	(39)
Period of coverage under the plan	10 years
Amounts recognized in the consolidated balance sheets consisted
Accrued expenses and other current liabilities	$ 180	140
Retiree health benefit obligation	6,240	6,809
Amounts recognized in the consolidated balance sheets	$ 6,420	$ 6,949
Salaried Pension Plan
Period of service of employees	12 years