Significant Accounting Policies - Basic and Diluted Earnings Per Common Share (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Net earnings attributable to Martin Marietta	$ 65.3	$ 25.9
Less: Distributed and undistributed earnings attributable to unvested awards	0.1
Basic and diluted net earnings available to common shareholders attributable to Martin Marietta	$ 65.2	$ 25.9
Basic weighted-average common shares outstanding	62.3	62.3
Effect of dilutive employee and director awards	0.2	0.2
Diluted weighted-average common shares outstanding	62.5	62.5