Significant Accounting Policies - Basic and Diluted Earnings Per Common Share (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net earnings attributable to Martin Marietta	$ 254.6	$ 294.4	$ 545.7	$ 538.0
Less: Distributed and undistributed earnings attributable to unvested awards		0.3		0.5
Basic and diluted net earnings available to common shareholders attributable to Martin Marietta	$ 254.6	$ 294.1	$ 545.7	$ 537.5
Basic weighted-average common shares outstanding	62.4	62.3	62.4	62.3
Effect of dilutive employee and director awards	0.2	0.1	0.2	0.1
Diluted weighted-average common shares outstanding	62.6	62.4	62.6	62.4