Segment Information - Financial information for segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 446.9	$ 445.0	$ 434.1	$ 410.9	$ 374.4	$ 378.6	$ 353.0	$ 354.4	$ 1,736.9	$ 1,460.4	$ 1,462.4
Operating Profit (Loss)	30.5	$ 42.8	$ 42.3	$ 37.3	26.7	$ 6.4	$ 30.6	$ 31.2	152.9	94.9	131.7
Assets	2,302.5				2,172.2				2,302.5	2,172.2	1,602.5
Depreciation, Depletion, & Amortization									85.8	67.6	65.7
Capital Expenditures									85.4	44.8	82.4
Total
Segment Reporting Information [Line Items]
Revenues									1,742.0	1,463.8	1,466.3
Operating Profit (Loss)									200.2	127.3	171.0
Assets	2,036.4				2,051.0				2,036.4	2,051.0	1,577.5
Depreciation, Depletion, & Amortization									82.2	67.1	65.7
Capital Expenditures									77.0	43.5	82.4
Corporate
Segment Reporting Information [Line Items]
Revenues									0.0	0.0	0.0
Operating Profit (Loss)									(47.3)	(32.1)	(39.3)
Assets	266.1				121.2				266.1	121.2	25.0
Depreciation, Depletion, & Amortization									3.6	0.5	0.0
Capital Expenditures									8.4	1.3	0.0
Construction Products Group
Segment Reporting Information [Line Items]
Assets	785.0				769.8				785.0	769.8	391.2
Depreciation, Depletion, & Amortization									38.0	21.9	18.4
Capital Expenditures									30.2	17.2	48.9
Construction Products Group | Total
Segment Reporting Information [Line Items]
Revenues									439.7	292.3	258.9
Operating Profit (Loss)									52.7	50.4	53.7
Energy Equipment Group
Segment Reporting Information [Line Items]
Assets	934.9				976.2				934.9	976.2	928.8
Depreciation, Depletion, & Amortization									27.9	29.7	30.2
Capital Expenditures									25.0	16.0	27.7
Energy Equipment Group | Total
Segment Reporting Information [Line Items]
Revenues									836.6	780.1	844.1
Operating Profit (Loss)									100.7	28.6	78.4
Transportation Products Group
Segment Reporting Information [Line Items]
Assets	316.5				305.0				316.5	305.0	257.5
Depreciation, Depletion, & Amortization									16.3	15.5	17.1
Capital Expenditures									21.8	10.3	5.8
Transportation Products Group | Total
Segment Reporting Information [Line Items]
Revenues									465.7	391.4	363.3
Operating Profit (Loss)									46.8	48.4	39.0
Other
Segment Reporting Information [Line Items]
Assets					0.0					0.0	0.0
Depreciation, Depletion, & Amortization										0.0	0.0
Capital Expenditures										0.0	0.0
Other | Total
Segment Reporting Information [Line Items]
Revenues										0.0	0.0
Operating Profit (Loss)										(0.1)	(0.1)
Construction aggregates | Construction Products Group
Segment Reporting Information [Line Items]
Revenues									364.7	217.9	204.9
Other | Construction Products Group
Segment Reporting Information [Line Items]
Revenues									75.0	74.4	54.0
Other | Energy Equipment Group
Segment Reporting Information [Line Items]
Revenues									211.2	197.2	192.0
Wind towers and utility structures | Energy Equipment Group
Segment Reporting Information [Line Items]
Revenues									625.4	582.9	652.1
Inland barge | Transportation Products Group
Segment Reporting Information [Line Items]
Revenues									293.9	170.2	157.9
Steel components | Transportation Products Group
Segment Reporting Information [Line Items]
Revenues									171.8	221.2	205.4
Eliminations
Segment Reporting Information [Line Items]
Assets	$ 0.0				$ 0.0				0.0	0.0	0.0
Depreciation, Depletion, & Amortization									0.0	0.0	0.0
Capital Expenditures									0.0	0.0	0.0
Eliminations | Intersegment
Segment Reporting Information [Line Items]
Revenues									(5.1)	(3.4)	(3.9)
Operating Profit (Loss)									$ 0.0	$ (0.3)	$ 0.0