Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Remeasurement of deferred tax assets and liabilities		$ 1,000
Income tax liability, transition toll tax					$ 6,100
Income before income taxes
Income before income taxes	$ 37,496	37,994	$ 37,244
Provision for income taxes
Federal	3,165	9,914	12,216
State	2,290	2,469	2,286
Canadian	50	(262)	398
Total current	5,505	12,121	14,900
Deferred taxes	4,014	3,753	60
Total taxes on income	$ 9,519	$ 15,874	$ 14,960
Tax rate reconciliation
Statutory U.S. federal tax rate	21.00%	35.00%	35.00%
State income taxes, net of federal benefit	5.70%	4.90%	4.00%
Effect of Canadian income taxes	0.10%	0.30%	(0.20%)
Nondeductible expenses	2.10%	2.20%	2.20%
ESOP dividend deduction	(0.30%)	(0.60%)	(0.70%)
Uncertain tax adjustments and audit settlement	(3.80%)	(0.20%)	0.20%
Income tax reform - deferred rate remeasurement	0.00%	2.50%	0.00%
All other, net	0.60%	(2.30%)	(0.30%)
Effective income tax rate	25.40%	41.80%	40.20%
Noncurrent deferred tax assets and liabilities, net
Self-insurance accruals				$ 17,579	14,186
Accrued compensated absences				1,565	1,747
Accrued expenses and other liabilities				742	798
Accrued stock compensation				1,772	1,571
Defined benefit pension plans				442	2,289
Foreign tax credit carryforward				2,117	605
Other future deductible amounts, net				3,271	3,661
Deferred tax assets, gross,noncurrent				27,488	24,857
Less deferred tax asset valuation allowance				1,683	0
Deferred tax assets, net--noncurrent				25,805	24,857
Intangibles				1,178	969
Prepaid expenses				3,160	2,300
Property and equipment				20,894	16,773
Deferred Tax Liabilities, Gross, Noncurrent				25,232	20,042
Net deferred tax assets--noncurrent				573	4,815
Undistributed earnings of foreign subsidiaries				28,856
Unrecognized benefits and related interest and penalties
Unrecognized tax benefits	$ 2,581	$ 2,532	$ 2,557	1,325	2,581
Portion, if recognized, would reduce tax expense and effective tax rate				599	1,948
Accrued interest on unrecognized tax benefits				$ 35	$ 128
Reconciliation of unrecognized tax benefits
Balance, beginning of year	2,581	2,532	2,557
Additions based on tax positions related to the current year	224	650	402
Additions for tax positions of prior years	20	0	51
Reductions for tax positions of prior years	(1,500)	(21)	(39)
Lapses in statutes of limitations	0	(580)	(439)
Balance, end of year	1,325	2,581	2,532
United States
Income before income taxes
Income before income taxes	37,377	38,800	35,867
Canada
Income before income taxes
Income before income taxes	$ 119	$ (806)	$ 1,377