Other intangible assets (Details 2) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 8,506,000	$ 11,266,000	$ 8,506,000	$ 11,266,000	$ 9,840,000
Amortization of intangible assets	700,000	1,000,000	1,400,000	1,800,000
Estimated amortization expense for amortizable intangible assets [Abstract]
2015	2,500,000		2,500,000
2016	2,200,000		2,200,000
2017	1,900,000		1,900,000
2018	1,000,000		1,000,000
2019	900,000		900,000
Thereafter	1,400,000		1,400,000
Customer relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	20,975,000	21,310,000	20,975,000	21,310,000	21,310,000
Intangible assets, accumulated amortization	(16,065,000)	(14,734,000)	(16,065,000)	(14,734,000)	(15,556,000)
Intangible assets, net (excluding goodwill)	4,910,000	6,576,000	4,910,000	6,576,000	5,754,000
Noncompete agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	4,409,000	5,080,000	4,409,000	5,080,000	5,080,000
Intangible assets, accumulated amortization	(3,581,000)	(3,936,000)	(3,581,000)	(3,936,000)	(4,098,000)
Intangible assets, net (excluding goodwill)	828,000	1,144,000	828,000	1,144,000	982,000
Other intangible assets [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	8,300,000	10,921,000	8,300,000	10,921,000	10,921,000
Intangible assets, accumulated amortization	(5,532,000)	(7,375,000)	(5,532,000)	(7,375,000)	(7,817,000)
Intangible assets, net (excluding goodwill)	$ 2,768,000	$ 3,546,000	$ 2,768,000	$ 3,546,000	$ 3,104,000