CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS Percentage	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues
Total revenue (as a percent)	100.00%	100.00%	100.00%	100.00%
Cost of revenues
Labor and other operating expenses (as a percent)	64.50%	64.70%	66.30%	65.10%
Depreciation, depletion and amortization (as a percent)	11.00%	11.10%	11.40%	11.40%
Total cost of revenues (as a percent)	75.50%	75.80%	77.60%	76.50%
Gross profit (as a percent)	24.50%	24.20%	22.40%	23.50%
Selling, general and administrative expenses (as a percent)	6.50%	6.90%	6.80%	6.80%
Operating profit (as a percent)	18.00%	17.30%	15.60%	16.70%
Other (income) expense
Interest expense (as a percent)	0.20%	0.10%	0.20%	0.20%
Interest and other income, net (as a percent)	(1.20%)	(0.60%)	(1.10%)	(0.60%)
Total other (income) expense (as a percent)	(1.00%)	(0.50%)	(0.90%)	(0.40%)
Income before income tax expense (as a percent)	19.00%	17.80%	16.50%	17.10%
Income tax expense (as a percent)	2.10%	3.30%	1.90%	3.50%
Net income (as a percent)	16.90%	14.50%	14.60%	13.60%
Lime and limestone operations
Revenues
Total revenue (as a percent)	98.30%	98.50%	98.30%	98.40%
Natural gas interests
Revenues
Total revenue (as a percent)	1.70%	1.50%	1.70%	1.60%