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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Net earnings attributable to Martin Marietta	$ 294.4	$ 248.6	$ 538.0	$ 480.9
Less: Distributed and undistributed earnings attributable to unvested awards	0.3	0.3	0.5	0.9
Basic and diluted net earnings available to common shareholders attributable to Martin Marietta	$ 294.1	$ 248.3	$ 537.5	$ 480.0
Basic weighted-average common shares outstanding	62.3	62.5	62.3	62.6
Effect of dilutive employee and director awards	0.1	0.2	0.1	0.1
Diluted weighted-average common shares outstanding	62.4	62.7	62.4	62.7