Significant Accounting Policies - Comprehensive Earnings Attributable to Martin Marietta Materials Incorporated (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net earnings attributable to Martin Marietta	$ 254.6	$ 294.4	$ 545.7	$ 538.0
Other comprehensive earnings, net of tax	1.6	3.6	7.0	7.2
Earnings attributable to Martin Marietta Materials, Inc.	$ 256.2	$ 298.0	$ 552.7	$ 545.2