Accounting Policies - Basic and Diluted Earnings Per Common Share (Detail) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net earnings attributable to Martin Marietta	$ 721.0	$ 611.9	$ 470.0
Less: distributed and undistributed earnings attributable to unvested participating securities	0.6	0.9	0.8
Basic and diluted net earnings attributable to common shareholders attributable to Martin Marietta	$ 720.4	$ 611.0	$ 469.2
Basic weighted-average common shares outstanding	62.3	62.5	62.9
Effect of dilutive employee and director awards	0.1	0.2	0.2
Diluted weighted-average common shares outstanding	62.4	62.7	63.1