Significant Accounting Policies - Comprehensive Earnings Attributable to Martin Marietta Materials Incorporated (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net earnings attributable to Martin Marietta	$ 217.6	$ 189.5	$ 243.5	$ 232.3
Other comprehensive earnings, net of tax	3.2	3.0	3.6	6.3
Earnings attributable to Martin Marietta Materials, Inc.	$ 220.8	$ 192.5	$ 247.1	$ 238.6