CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS Percentage	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenues
Revenues (as a percent)	100.00%	100.00%
Cost of revenues
Labor and other operating expenses (as a percent)	59.00%	62.30%
Depreciation, depletion and amortization (as a percent)	12.70%	12.00%
Total cost of revenues (as a percent)	71.70%	74.30%
Gross profit (as a percent)	28.30%	25.70%
Selling, general and administrative expenses (as a percent)	7.40%	8.40%
Operating profit (as a percent)	20.90%	17.30%
Other expense (income)
Interest expense (as a percent)	0.10%	0.20%
Interest and other income, net (as a percent)	(0.10%)	(0.70%)
Total other expense (income) (as a percent)	0.00%	(0.50%)
Income before income tax expense (as a percent)	20.90%	17.80%
Income tax expense (as a percent)	4.00%	3.40%
Net income (as a percent)	16.90%	14.40%