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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net earnings attributable to Martin Marietta	$ 217.6	$ 189.5	$ 243.5	$ 232.3
Less: Distributed and undistributed earnings attributable to unvested awards	0.2	0.4	0.2	0.5
Basic and diluted net earnings available to common shareholders attributable to Martin Marietta	$ 217.4	$ 189.1	$ 243.3	$ 231.8
Basic weighted-average common shares outstanding	62.3	62.6	62.3	62.6
Effect of dilutive employee and director awards		0.1	0.1	0.1
Diluted weighted-average common shares outstanding	62.3	62.7	62.4	62.7