Other intangible assets (Details 2) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 4,190,000	$ 4,785,000	$ 4,190,000	$ 4,785,000	$ 3,837,000
Amortization of intangible assets	300,000	600,000	700,000	1,200,000
Estimated amortization expense for amortizable intangible assets [Abstract]
2018	1,400,000		1,400,000
2019	1,100,000		1,100,000
2020	600,000		600,000
2021	400,000		400,000
2022	400,000		400,000
Thereafter	1,000,000.0		1,000,000.0
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	15,587,000	15,745,000	15,587,000	15,745,000	15,248,000
Intangible assets, less accumulated amortization	13,191,000	13,302,000	13,191,000	13,302,000	13,382,000
Intangible assets, net (excluding goodwill)	2,396,000	2,443,000	2,396,000	2,443,000	1,866,000
Noncompete agreements
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	2,546,000	2,430,000	2,546,000	2,430,000	2,430,000
Intangible assets, less accumulated amortization	1,877,000	1,732,000	1,877,000	1,732,000	1,805,000
Intangible assets, net (excluding goodwill)	669,000	698,000	669,000	698,000	625,000
Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	6,458,000	7,086,000	6,458,000	7,086,000	6,990,000
Intangible assets, less accumulated amortization	5,333,000	5,442,000	5,333,000	5,442,000	5,644,000
Intangible assets, net (excluding goodwill)	$ 1,125,000	$ 1,644,000	$ 1,125,000	$ 1,644,000	$ 1,346,000