CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS Percentage	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues
Total revenues (as a percent)	100.00%	100.00%	100.00%	100.00%
Cost of revenues
Labor and other operating expenses (as a percent)	67.20%	62.70%	66.50%	64.30%
Depreciation, depletion and amortization (as a percent)	12.40%	11.50%	11.70%	11.40%
Total cost of revenues (as a percent)	79.60%	74.20%	78.20%	75.70%
Gross profit (as a percent)	20.40%	25.80%	21.80%	24.30%
Selling, general and administrative expenses (as a percent)	7.90%	7.20%	7.20%	6.90%
Operating profit (as a percent)	12.50%	18.60%	14.60%	17.40%
Other (income) expense
Interest expense (as a percent)	0.20%	0.10%	0.20%	0.20%
Interest and other income, net (as a percent)	(1.40%)	(0.70%)	(1.20%)	(0.60%)
Total other (income) expense (as a percent)	(1.20%)	(0.60%)	(1.00%)	(0.40%)
Income before income tax expense (as a percent)	13.70%	19.20%	15.60%	17.80%
Income tax expense (as a percent)	0.80%	3.90%	1.50%	3.60%
Net income (as a percent)	12.90%	15.30%	14.10%	14.20%
Lime and limestone operations
Revenues
Total revenues (as a percent)	98.40%	98.60%	98.30%	98.50%
Natural gas interests
Revenues
Total revenues (as a percent)	1.60%	1.40%	1.70%	1.50%