CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS Percentage	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 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)	56.40%	59.40%	59.40%	63.10%
Depreciation, depletion and amortization (as a percent)	11.20%	9.60%	11.90%	10.40%
Total cost of revenues (as a percent)	67.60%	69.00%	71.30%	73.50%
Gross profit (as a percent)	32.40%	31.00%	28.70%	26.50%
Selling, general and administrative expenses (as a percent)	6.70%	6.80%	7.50%	6.80%
Operating profit (as a percent)	25.70%	24.20%	21.20%	19.70%
Other expense (income)
Interest expense (as a percent)	0.10%	0.10%	0.20%	0.20%
Interest and other income, net (as a percent)	(0.10%)	(1.10%)	(0.40%)	(1.20%)
Total other expense (income) (as a percent)	0.00%	(1.00%)	(0.20%)	(1.00%)
Income before income tax expense (as a percent)	25.70%	25.20%	21.40%	20.70%
Income tax expense (as a percent)	4.40%	2.50%	3.90%	3.20%
Net income (as a percent)	21.30%	22.70%	17.50%	17.50%