CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS Percentage	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 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)	55.30%	59.80%	57.00%	61.10%
Depreciation, depletion and amortization (as a percent)	10.50%	12.60%	11.50%	12.30%
Total cost of revenues (as a percent)	65.80%	72.40%	68.50%	73.40%
Gross profit (as a percent)	34.20%	27.60%	31.50%	26.60%
Selling, general and administrative expenses (as a percent)	6.00%	7.70%	6.60%	8.00%
Operating profit (as a percent)	28.20%	19.90%	24.90%	18.60%
Other expense (income)
Interest expense (as a percent)	0.20%	0.20%	0.10%	0.20%
Interest and other income, net (as a percent)	(0.20%)	(0.30%)	(0.10%)	(0.50%)
Total other expense (income) (as a percent)	0.00%	(0.10%)	0.00%	(0.30%)
Income before income tax expense (as a percent)	28.20%	20.00%	24.90%	18.90%
Income tax expense (as a percent)	5.60%	3.80%	4.90%	3.60%
Net income (as a percent)	22.60%	16.20%	20.00%	15.30%