Employee Retirement Plans - Reconciliations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in plan assets:
Fair value of plan assets at beginning of year	$ 26,378
Fair value of plan assets at end of year	29,223	$ 26,378
Amounts recognized in the consolidated balance sheets consisted
Retiree health benefit obligation	7,193	6,656
Pension plan
Components of net periodic cost:
Service cost	321	257	$ 216
Interest cost	1,639	1,489	1,496
Expected return on plan assets	(1,824)	(1,630)	(1,631)
Amortization of net (gain) loss	724	1,021	203
Net periodic pension cost	860	1,137	284
Employer contributions during the period	1,711	1,775
Change in projected benefit obligation:
Benefit obligation at beginning of year	37,217	38,362
Actuarial (gain) loss	1,469	(1,662)
Benefits paid	(1,239)	(1,229)
Benefit obligation at end of year	39,407	37,217	38,362
Change in plan assets:
Fair value of plan assets at beginning of year	26,378	26,046
Actual return (loss) on plan assets	2,373	(214)
Benefits paid	(1,239)	(1,229)
Fair value of plan assets at end of year	29,223	26,378	26,046
Defined Benefit Plan, Funded Status of Plan, Total	(10,184)	(10,839)
Salaried Pension Plan
Accumulated benefit obligation	38,799	36,749
Pension plan | Minimum
Components of net periodic cost:
Employer contributions during the period		411
Other postretirement benefit cost
Components of net periodic cost:
Service cost	210	229	159
Interest cost	278	256	213
Amortization of net (gain) loss	(127)	(69)	(398)
Net periodic pension cost	361	416	(26)
Change in projected benefit obligation:
Benefit obligation at beginning of year	6,896	7,044
Participant contributions	38	45
Actuarial (gain) loss	53	(508)
Benefits paid	(142)	(170)
Benefit obligation at end of year	7,333	6,896	$ 7,044
Change in plan assets:
Benefits paid	$ (142)	(170)
Period of coverage under the plan	10 years
Amounts recognized in the consolidated balance sheets consisted
Accrued expenses and other current liabilities	$ 140	240
Retiree health benefit obligation	7,193	6,656
Amounts recognized in the consolidated balance sheets	$ 7,333	$ 6,896
Salaried Pension Plan
Period of service of employees	12 years