Other intangible assets (Details 2) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 7,160,000	$ 8,506,000	$ 7,160,000	$ 8,506,000	$ 7,342,000
Amortization of intangible assets	600,000	700,000	1,300,000	1,400,000
Estimated amortization expense for amortizable intangible assets [Abstract]
2016	2,500,000		2,500,000
2017	2,200,000		2,200,000
2018	1,200,000		1,200,000
2019	1,000,000		1,000,000
2020	500,000		500,000
Thereafter	1,100,000		1,100,000
Customer relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	17,145,000	20,975,000	17,145,000	20,975,000	20,975,000
Intangible assets, accumulated amortization	(13,108,000)	(16,065,000)	(13,108,000)	(16,065,000)	(16,845,000)
Intangible assets, net (excluding goodwill)	4,037,000	4,910,000	4,037,000	4,910,000	4,130,000
Noncompete agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	2,430,000	4,409,000	2,430,000	4,409,000	4,409,000
Intangible assets, accumulated amortization	(1,585,000)	(3,581,000)	(1,585,000)	(3,581,000)	(3,655,000)
Intangible assets, net (excluding goodwill)	845,000	828,000	845,000	828,000	754,000
Other intangible assets [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	7,764,000	8,300,000	7,764,000	8,300,000	8,304,000
Intangible assets, accumulated amortization	(5,486,000)	(5,532,000)	(5,486,000)	(5,532,000)	(5,846,000)
Intangible assets, net (excluding goodwill)	$ 2,278,000	$ 2,768,000	$ 2,278,000	$ 2,768,000	$ 2,458,000