Accounts Receivable, Net and Supplemental Balance Sheet Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Accounts receivable, net
Accounts receivable	$ 263,426,000	$ 242,427,000
Unbilled receivables (1)	99,485,000	82,605,000
Accounts receivable, gross, current	362,911,000	325,032,000
Less allowances for credit losses	2,441,000	3,222,000
Accounts receivable, net	360,470,000	321,810,000
Other current assets
Refundable income taxes	0	14,000
Mitigation bank credit inventory	28,385,000	6,351,000
Assets invested for self-insurance	20,959,000	24,828,000
Payroll taxes refundable	22,591,000	18,283,000
Other	302,000	2,396,000
Other current assets, total	72,237,000	51,872,000
Property, plant and equipment
Land and land improvements	28,177,000	26,023,000
Buildings and leasehold improvements	99,964,000	80,768,000
Equipment	684,562,000	663,207,000
Property and equipment	812,703,000	769,998,000
Less accumulated depreciation	500,025,000	501,459,000
Property and equipment, net	312,678,000	268,539,000
Interest Paid, Including Capitalized Interest, Operating and Investing Activities	4,961,000	2,049,000
Other assets, noncurrent
Investment--cost-method affiliate	1,405,000	1,258,000
Net deferred tax asset--noncurrent	6,001,000	6,828,000
Cloud computing arrangements	107,000	2,652,000
Other	4,362,000	4,603,000
Other assets, noncurrent, total	11,875,000	15,341,000
Accrued expenses
Employee compensation	40,656,000	35,536,000
Accrued compensated absences	14,483,000	13,034,000
Self-insured medical claims	2,309,000	2,806,000
Customer advances, deposits	1,275,000	7,736,000
Income taxes payable	1,135,000	6,573,000
Taxes, other than income	6,017,000	5,764,000
Other	16,069,000	6,005,000
Accrued expenses, total	81,944,000	77,454,000
Other liabilities, noncurrent
Pension and retirement plans	5,630,000	8,336,000
Other	6,096,000	8,184,000
Other liabilities, noncurrent, total	$ 11,726,000	$ 16,520,000