CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS Percentage	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenues
Revenues (as a percent)	100.00%	100.00%
Cost of revenues
Labor and other operating expenses (as a percent)	62.30%	66.20%
Depreciation, depletion and amortization (as a percent)	12.00%	10.70%
Total cost of revenues (as a percent)	74.30%	76.90%
Gross profit (as a percent)	25.70%	23.10%
Selling, general and administrative expenses (as a percent)	8.40%	7.10%
Operating profit (as a percent)	17.30%	16.00%
Other expense (income)
Interest expense (as a percent)	0.20%	0.20%
Interest and other income, net (as a percent)	(0.70%)	(1.30%)
Total other expense (income) (as a percent)	(0.50%)	(1.10%)
Income before income tax expense (as a percent)	17.80%	17.10%
Income tax expense (as a percent)	3.40%	3.50%
Net income (as a percent)	14.40%	13.60%