Significant Accounting Policies - Basic and Diluted Earnings Per Common Share (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Net earnings attributable to Martin Marietta	$ 225.8	$ 217.6	$ 291.1	$ 243.5
Less: Distributed and undistributed earnings attributable to unvested awards	0.2	0.2	0.2	0.2
Basic and diluted net earnings available to common shareholders attributable to Martin Marietta	$ 225.6	$ 217.4	$ 290.9	$ 243.3
Basic weighted-average common shares outstanding	62.4	62.3	62.4	62.3
Effect of dilutive employee and director awards	0.1		0.1	0.1
Diluted weighted-average common shares outstanding	62.5	62.3	62.5	62.4