Income Taxes - Summary of Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Statutory income tax rate	21.00%	21.00%	21.00%
Effect of statutory depletion	(2.80%)	(3.40%)	(3.40%)
State income taxes, net of federal tax benefit	2.60%	2.00%	2.80%
Railroad track maintenance credits	(1.30%)
Change in tax status of subsidiary		(1.70%)
Impact from 2017 Tax Cuts and Jobs Act			(3.30%)
Other items	(0.60%)	0.30%	1.20%
Effective income tax rate	18.90%	18.20%	18.30%