Organization and Nature of Business (Detail) $ in Thousands, T in Millions		3 Months Ended		9 Months Ended
	Sep. 18, 2020 USD ($)	Sep. 30, 2021 USD ($) T	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) T	Sep. 30, 2020 USD ($)
Business Acquisition [Line Items]
Processing capacity | T		7.1		7.1
Other income | $		$ 1,792	$ 80	$ 5,457	$ 162
Business Combination, Bargain Purchase, Gain Recognized, Amount | $	$ 39,600
Eagle Materials Holdings [Member] | BNSF [Member]
Business Acquisition [Line Items]
Processing capacity | T		1.6		1.6