Goodwill and other intangible assets (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Finite-lived intangible assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 9,840,000	$ 13,099,000
Amortization of intangible assets	3,200,000	4,000,000	3,800,000
Estimated amortization expense for amortizable intangible assets
2015	2,500,000
2016	2,100,000
2017	1,900,000
2018	1,000,000
2019	900,000
Thereafter	1,400,000
Customer relationships [Member]
Finite-lived intangible assets [Line Items]
Other amortizable intangible assets	21,310,000	21,310,000
Accumulated amortization	(15,556,000)	(13,726,000)
Intangible assets, net (excluding goodwill)	5,754,000	7,584,000
Noncompete agreements [Member]
Finite-lived intangible assets [Line Items]
Other amortizable intangible assets	5,080,000	6,186,000
Accumulated amortization	(4,098,000)	(4,840,000)
Intangible assets, net (excluding goodwill)	982,000	1,346,000
Other [Member}
Finite-lived intangible assets [Line Items]
Other amortizable intangible assets	10,921,000	10,995,000
Accumulated amortization	(7,817,000)	(6,826,000)
Intangible assets, net (excluding goodwill)	$ 3,104,000	$ 4,169,000