Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
Income before income taxes
Income before income taxes	$ 37,244	$ 35,258	$ 38,300
Provision for income taxes
Federal	12,216	13,337	13,159
State	2,286	2,487	2,524
Canadian	398	611	2,127
Total current	14,900	16,435	17,810
Deferred taxes	60	(2,975)	(2,679)
Total taxes on income	$ 14,960	$ 13,460	$ 15,131
Tax rate reconciliation
Statutory U.S. federal tax rate	35.00%	35.00%	35.00%
State income taxes, net of federal benefit	4.00%	3.70%	3.90%
Effect of Canadian income taxes	(0.20%)	(0.50%)	(1.70%)
Nondeductible expenses	2.20%	2.30%	1.90%
ESOP dividend deduction	(0.70%)	(0.80%)	(0.70%)
Uncertain tax adjustments and audit settlement	0.20%	1.50%	1.60%
Valuation allowance	(0.00%)	(2.20%)	(0.00%)
All other, net	(0.30%)	(0.80%)	(0.50%)
Effective income tax rate	40.20%	38.20%	39.50%
Current deferred tax assets and liabilities, net
Deferred income tax assets--noncurrent				$ 38,210	$ 36,479
Net deferred tax assets--noncurrent				11,129	11,521
Noncurrent deferred tax assets and liabilities, net
Self-insurance accruals				21,967	21,136
Accrued compensated absences				3,540	3,189
Accrued expenses and other liabilities				1,394	1,255
Accrued stock compensation				2,502	2,144
Defined benefit pension plans				4,019	3,721
Foreign tax credit carryforward				575	680
Other future deductible amounts, net				4,213	4,354
Deferred tax assets, net--noncurrent				38,210	36,479
Intangibles				297	150
Prepaid expenses				3,510	3,138
Property and equipment				23,274	21,670
Deferred tax liability, noncurrent				27,081	24,958
Net deferred tax assets--noncurrent				11,129	11,521
Undistributed earnings of foreign subsidiaries				28,983
Unrecognized benefits and related interest and penalties
Unrecognized tax benefits	$ 2,557	$ 1,949	$ 1,221	2,532	2,557
Portion, if recognized, would reduce tax expense and effective tax rate				2,053	1,981
Accrued interest on unrecognized tax benefits				$ 107	$ 115
Reconciliation of unrecognized tax benefits
Balance, beginning of year	2,557	1,949	1,221
Additions based on tax positions related to the current year	402	642	587
Additions for tax positions of prior years	51	52	234
Reductions for tax positions of prior years	(39)	(63)	(79)
Lapses in statutes of limitations	(439)	(23)	(14)
Balance, end of year	2,532	2,557	1,949
United States [Member]
Income before income taxes
Income before income taxes	35,867	32,807	30,380
Canada [Member]
Income before income taxes
Income before income taxes	$ 1,377	$ 2,451	$ 7,920