Goodwill and other intangible assets (Details 2) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-lived intangible assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 3,837,000	$ 5,925,000
Amortization of intangible assets	2,000,000.0	2,500,000	$ 2,500,000
Estimated amortization expense for amortizable intangible assets
2018	1,300,000
2019	1,000,000.0
2020	500,000
2021	300,000
2022	200,000
Thereafter	500,000
Customer relationships [Member]
Finite-lived intangible assets [Line Items]
Intangible assets, gross	15,248,000	17,145,000
Intangible assets, less accumulated amortization	13,382,000	13,917,000
Intangible assets, net (excluding goodwill)	1,866,000	3,228,000
Noncompete agreements [Member]
Finite-lived intangible assets [Line Items]
Intangible assets, gross	2,430,000	2,430,000
Intangible assets, less accumulated amortization	1,805,000	1,658,000
Intangible assets, net (excluding goodwill)	625,000	772,000
Other [Member}
Finite-lived intangible assets [Line Items]
Intangible assets, gross	6,990,000	7,768,000
Intangible assets, less accumulated amortization	5,644,000	5,843,000
Intangible assets, net (excluding goodwill)	$ 1,346,000	$ 1,925,000