Goodwill and other intangible assets (Details 2) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Finite-lived intangible assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 7,342,000	$ 9,840,000
Amortization of intangible assets	2,500,000	3,200,000	$ 4,000,000
Estimated amortization expense for amortizable intangible assets
2016	2,200,000
2017	1,900,000
2018	1,000,000
2019	900,000
2020	300,000
Thereafter	1,000,000
Customer relationships [Member]
Finite-lived intangible assets [Line Items]
Other amortizable intangible assets	20,975,000	21,310,000
Accumulated amortization	(16,845,000)	(15,556,000)
Intangible assets, net (excluding goodwill)	4,130,000	5,754,000
Noncompete agreements [Member]
Finite-lived intangible assets [Line Items]
Other amortizable intangible assets	4,409,000	5,080,000
Accumulated amortization	(3,655,000)	(4,098,000)
Intangible assets, net (excluding goodwill)	754,000	982,000
Other [Member}
Finite-lived intangible assets [Line Items]
Other amortizable intangible assets	8,304,000	10,921,000
Accumulated amortization	(5,846,000)	(7,817,000)
Intangible assets, net (excluding goodwill)	$ 2,458,000	$ 3,104,000