Consolidated Statements of Comprehensive Earnings - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Consolidated Net Earnings	$ 612.0	$ 470.4	$ 713.4
Defined benefit pension and postretirement plans:
Net loss arising during period, net of tax of $(4.8), $(7.6) and $(2.6), respectively	(14.5)	(22.9)	(8.1)
Amortization of prior service credit, net of tax of $(0.2), $(0.5) and $(0.5), respectively	(0.6)	(1.5)	(0.8)
Amortization of actuarial loss, net of tax of $3.8, $3.2 and $5.3, respectively	11.7	9.5	8.5
Amount recognized in net periodic pension cost due to settlement, net of tax of $0.0, $0.7 and $0.0 respectively		2.2
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	(3.4)	(12.7)	(0.4)
Foreign currency translation gain (loss)	1.2	(2.1)	1.2
Amortization of terminated value of forward starting interest rate swap agreements into interest expense, net of tax of $0.0, $0.2 and $0.6, respectively		0.3	0.8
Other Comprehensive Income (Loss), Net of Tax	(2.2)	(14.5)	1.6
Consolidated comprehensive earnings	609.8	455.9	715.0
Less: Comprehensive earnings attributable to noncontrolling interests	0.1	0.4	0.1
Comprehensive Earnings Attributable to Martin Marietta	$ 609.7	$ 455.5	$ 714.9