Operations by Industry Segment and Geographic Area (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Segment Businessunit	Dec. 31, 2016 USD ($) Customer	Dec. 31, 2015 USD ($)
Operations by Industry Segment and Geographic Area [Abstract]
Number of reportable segments | Segment	3
Sales [Abstract]
Revenues	$ 1,184,739	$ 1,147,431	$ 983,157
Interest expense	840	1,395	1,611
Depreciation and amortization	25,802	24,813	24,078
Income taxes	19,627	32,107	20,007
Profit (loss)	37,645	54,512	31,566
Assets	1,919,793	1,864,048	1,627,514
Capital expenditures	20,762	27,084	21,288
Net income attributable to controlling interest [Abstract]
Net loss attributable to non-controlling interest	205	171	831
Net income (loss) attributable to controlling interest	37,795	55,159	32,797
Assets [Abstract]
Total assets	$ 889,579	$ 843,601	777,353
Infrastructure Group [Member]
Segment Reporting Information [Line Items]
Number of business units | Businessunit	5
Number of business units which design, engineer, manufacture and market product lines | Businessunit	3
Number of business units that operate as Company-owned dealers | Businessunit	2
Number of major customers | Customer		1
Infrastructure Group [Member] | Pellet Plant [Member]
Segment Reporting Information [Line Items]
Revenue percentage of total company sales	0.70%	11.80%
Sales [Abstract]
Revenues	$ 7,987	$ 135,187
Aggregate and Mining Group [Member]
Segment Reporting Information [Line Items]
Number of business units | Businessunit	8
Energy Group [Member]
Segment Reporting Information [Line Items]
Number of business units | Businessunit	6
Reportable Segments [Member]
Net income attributable to controlling interest [Abstract]
Total profit	$ 78,608	110,504	68,189
Assets [Abstract]
Total assets	1,529,493	1,446,697	1,321,003
Reportable Segments [Member] | Infrastructure Group [Member]
Sales [Abstract]
Revenues	553,691	608,908	428,737
Interest expense	49	31	258
Depreciation and amortization	7,581	7,205	6,907
Income taxes	1,318	3,033	1,224
Profit (loss)	26,641	71,482	33,890
Assets	666,651	657,225	567,936
Capital expenditures	7,424	14,451	8,043
Reportable Segments [Member] | Aggregate and Mining Group [Member]
Sales [Abstract]
Revenues	403,720	359,760	370,813
Interest expense	634	948	1,005
Depreciation and amortization	9,363	10,033	10,719
Income taxes	462	664	764
Profit (loss)	35,748	34,877	30,690
Assets	558,684	518,351	496,089
Capital expenditures	9,194	7,437	8,807
Reportable Segments [Member] | Energy Group [Member]
Sales [Abstract]
Revenues	227,328	178,763	183,607
Interest expense	9	4	10
Depreciation and amortization	7,904	6,655	5,553
Income taxes	491	437	(129)
Profit (loss)	16,219	4,145	3,609
Assets	304,158	271,121	256,978
Capital expenditures	3,540	5,018	4,049
Reportable Segments [Member] | Corporate Group [Member]
Sales [Abstract]
Revenues	0	0	0
Interest expense	148	412	338
Depreciation and amortization	954	920	899
Income taxes	17,356	27,973	18,148
Profit (loss)	(40,963)	(55,992)	(36,623)
Assets	390,300	417,351	306,511
Capital expenditures	604	178	389
Corporate, Non-Segment [Member]
Net income attributable to controlling interest [Abstract]
Total profit	(40,963)	(55,992)	(36,623)
Assets [Abstract]
Total assets	390,300	417,351	306,511
Segment Reconciling Items [Member]
Assets [Abstract]
Elimination of investment in subsidiaries	(303,209)	(272,766)	(223,500)
Other eliminations	(2,057)	(52,292)	(35,331)
Intersegment Eliminations [Member]
Sales [Abstract]
Revenues	67,051	76,934	67,658
Net income attributable to controlling interest [Abstract]
Total profit	(55)	476	400
Assets [Abstract]
Elimination of intercompany profit in inventory	(7,075)	(7,020)	(7,496)
Elimination of intercompany receivables	(717,873)	(688,369)	(583,834)
Intersegment Eliminations [Member] | Infrastructure Group [Member]
Sales [Abstract]
Revenues	25,965	16,957	22,947
Intersegment Eliminations [Member] | Aggregate and Mining Group [Member]
Sales [Abstract]
Revenues	16,209	35,031	28,701
Intersegment Eliminations [Member] | Energy Group [Member]
Sales [Abstract]
Revenues	24,877	24,946	16,010
Intersegment Eliminations [Member] | Corporate Group [Member]
Sales [Abstract]
Revenues	$ 0	$ 0	$ 0