Goodwill and other intangible assets (Details 2) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Finite-lived intangible assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 5,925,000	$ 7,342,000
Amortization of intangible assets	2,500,000	2,500,000	$ 3,200,000
Estimated amortization expense for amortizable intangible assets
2017	2,200,000
2018	1,200,000
2019	1,000,000
2020	500,000
2021	200,000
Thereafter	800,000
Customer relationships [Member]
Finite-lived intangible assets [Line Items]
Intangible assets, gross	17,145,000	20,975,000
Intangible assets, less accumulated amortization	13,917,000	16,845,000
Intangible assets, net (excluding goodwill)	3,228,000	4,130,000
Noncompete agreements [Member]
Finite-lived intangible assets [Line Items]
Intangible assets, gross	2,430,000	4,409,000
Intangible assets, less accumulated amortization	1,658,000	3,655,000
Intangible assets, net (excluding goodwill)	772,000	754,000
Other [Member}
Finite-lived intangible assets [Line Items]
Intangible assets, gross	7,768,000	8,304,000
Intangible assets, less accumulated amortization	5,843,000	5,846,000
Intangible assets, net (excluding goodwill)	$ 1,925,000	$ 2,458,000