Contingencies - Narrative (Details)			3 Months Ended		9 Months Ended
	Jun. 23, 2015 USD ($)	Jun. 09, 2015 USD ($)	Sep. 30, 2017 USD ($) state	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) state lawsuit county	Sep. 30, 2016 USD ($)	Jun. 16, 2016 briefs	Mar. 28, 2016 briefs	Mar. 11, 2015 guard_rail	Oct. 20, 2014 crash_test
Loss Contingencies [Line Items]
Revenues			$ 973,600,000	$ 1,111,700,000	$ 2,756,400,000	$ 3,484,500,000
Accrued liabilities
Loss Contingencies [Line Items]
Total accruals			12,200,000		12,200,000
Construction Products Group
Loss Contingencies [Line Items]
Revenues			130,100,000	136,300,000	381,700,000	399,600,000
Highway Products Litigation
Loss Contingencies [Line Items]
Number of additional crash tests requested | crash_test										8
Number of additional crash tests passed, first installation height | crash_test										4
Number of additional crash tests passed, second installation height | crash_test										4
Number of additional crash tests requested passed | crash_test										8
Number of ET Plus devices field measured by FHWA engineers (more than) | guard_rail									1,000
Highway Products Litigation | ET Plus
Loss Contingencies [Line Items]
Revenues			$ 1,000,000	$ 900,000	$ 3,300,000	$ 3,400,000
Number of states that have removed product from qualified products list | state			29		29
Highway Products Litigation | False Claims Act, USA
Loss Contingencies [Line Items]
Damages awarded by jury verdict		$ 175,000,000
Total amount of judgment entered by the District Court		682,400,000
Damages awarded by jury verdict, automatically trebled under the Act		525,000,000
Civil penalties included in judgment entered by the District Court		138,400,000
Costs and attorney's fees included in judgment entered by the District Court		$ 19,000,000
Amount of supersedeas bond posted	$ 686,000,000
Initial annual premium of supersedeas bond	$ 3,700,000
Number of amicus curiae briefs filed seeking judgment reversal | briefs								6
Number of amicus curiae briefs filed in opposition of Company's appeal | briefs							6
Total accruals			$ 0		$ 0
Highway Products Litigation | State, county, and municipal actions
Loss Contingencies [Line Items]
Total accruals			0		$ 0
Number of additional separate state qui tam actions filed | lawsuit					9
Number of class action lawsuits | lawsuit					2
Number of other counties claimed as parties to class action suit | county					101
Compensatory damages sought					$ 400,000,000
Punitive damages sought					100,000,000
Highway Products Litigation | Class Action, Shareholder
Loss Contingencies [Line Items]
Total accruals			0		0
Environmental and workplace matters
Loss Contingencies [Line Items]
Total accruals			3,500,000		3,500,000
Minimum
Loss Contingencies [Line Items]
Range of possible loss			3,200,000		3,200,000
Maximum
Loss Contingencies [Line Items]
Range of possible loss			$ 19,000,000		$ 19,000,000