Accounting Policies - Basic and Diluted Earnings Per Common Share (Detail) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net earnings attributable to Martin Marietta	$ 611.9	$ 470.0	$ 713.3
Less: Distributed and undistributed earnings attributable to unvested participating securities	0.9	0.8	2.0
Basic and diluted net earnings attributable to common shareholders attributable to Martin Marietta	$ 611.0	$ 469.2	$ 711.3
Basic weighted-average common shares outstanding	62.5	62.9	62.9
Effect of dilutive employee and director awards	0.2	0.2	0.3
Diluted weighted-average common shares outstanding	62.7	63.1	63.2