Significant Accounting Policies - Comprehensive Earnings Attributable to Martin Marietta Materials Incorporated (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net earnings attributable to Martin Marietta	$ 294.4	$ 248.6	$ 538.0	$ 480.9
Other comprehensive earnings, net of tax	3.6	2.5	7.2	8.8
Earnings attributable to Martin Marietta Materials, Inc.	$ 298.0	$ 251.1	$ 545.2	$ 489.7