Other intangible assets (Details 2) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 15,511,000	$ 4,318,000	$ 15,511,000	$ 4,318,000	$ 10,815,000
Amortization of intangible assets	500,000	300,000	1,600,000	900,000
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	18,011,000	15,920,000	18,011,000	15,920,000	22,720,000
Intangible assets, less accumulated amortization	5,823,000	13,342,000	5,823,000	13,342,000	13,535,000
Intangible assets, net (excluding goodwill)	12,188,000	2,578,000	12,188,000	2,578,000	9,185,000
Noncompete agreements
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	3,419,000	2,606,000	3,419,000	2,606,000	2,605,000
Intangible assets, less accumulated amortization	1,868,000	1,911,000	1,868,000	1,911,000	1,956,000
Intangible assets, net (excluding goodwill)	1,551,000	695,000	1,551,000	695,000	649,000
Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	7,488,000	6,458,000	7,488,000	6,458,000	6,458,000
Intangible assets, less accumulated amortization	5,716,000	5,413,000	5,716,000	5,413,000	5,477,000
Intangible assets, net (excluding goodwill)	$ 1,772,000	$ 1,045,000	1,772,000	$ 1,045,000	$ 981,000
Construction materials and contracting [Member] | Series of Individually Immaterial Business Acquisitions
Finite-Lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired			$ 6,300,000