CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS Percentage	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues
Revenues (as a percent)	100.00%	100.00%	100.00%	100.00%
Cost of revenues
Labor and other operating expenses (as a percent)	57.30%	56.40%	57.10%	59.40%
Depreciation, depletion and amortization (as a percent)	9.60%	11.20%	10.80%	11.90%
Total cost of revenues (as a percent)	66.90%	67.60%	67.90%	71.30%
Gross profit (as a percent)	33.10%	32.40%	32.10%	28.70%
Selling, general and administrative expenses (as a percent)	6.00%	6.70%	6.40%	7.50%
Operating profit (as a percent)	27.10%	25.70%	25.70%	21.20%
Other expense (income)
Interest expense (as a percent)	0.10%	0.10%	0.10%	0.20%
Interest and other income, net (as a percent)	(0.30%)	(0.10%)	(0.20%)	(0.40%)
Total other expense (income) (as a percent)	(0.20%)	0.00%	(0.10%)	(0.20%)
Income before income tax expense (as a percent)	27.30%	25.70%	25.80%	21.40%
Income tax expense (as a percent)	5.70%	4.40%	5.20%	3.90%
Net income (as a percent)	21.60%	21.30%	20.60%	17.50%