CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS Percentage	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenues
Revenues (as a percent)	100.00%	100.00%	100.00%	100.00%
Cost of revenues
Labor and other operating expenses (as a percent)	59.80%	64.20%	61.10%	65.20%
Depreciation, depletion and amortization (as a percent)	12.60%	10.80%	12.30%	10.80%
Total cost of revenues (as a percent)	72.40%	75.00%	73.40%	76.00%
Gross profit (as a percent)	27.60%	25.00%	26.60%	24.00%
Selling, general and administrative expenses (as a percent)	7.70%	6.80%	8.00%	6.90%
Operating profit (as a percent)	19.90%	18.20%	18.60%	17.10%
Other expense (income)
Interest expense (as a percent)	0.20%	0.20%	0.20%	0.20%
Interest and other income, net (as a percent)	(0.30%)	(1.30%)	(0.50%)	(1.30%)
Total other expense (income) (as a percent)	(0.10%)	(1.10%)	(0.30%)	(1.10%)
Income before income tax expense (as a percent)	20.00%	19.30%	18.90%	18.20%
Income tax expense (as a percent)	3.80%	3.80%	3.60%	3.70%
Net income (as a percent)	16.20%	15.50%	15.30%	14.50%