Goodwill and other intangible assets (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Finite-lived intangible assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 17,129,000	$ 20,843,000
Amortization of intangible assets	3,800,000	3,700,000	4,200,000
Estimated amortization expense for amortizable intangible assets
2013	3,700,000
2014	3,500,000
2015	2,600,000
2016	2,200,000
2017	1,900,000
Thereafter	3,200,000
Customer relationships [Member]
Finite-lived intangible assets [Line Items]
Other amortizable intangible assets	21,310,000	21,702,000
Accumulated amortization	(11,701,000)	(10,392,000)
Intangible assets, net (excluding goodwill)	9,609,000	11,310,000
Noncompete agreements [Member]
Finite-lived intangible assets [Line Items]
Other amortizable intangible assets	7,236,000	7,685,000
Accumulated amortization	(5,326,000)	(5,371,000)
Intangible assets, net (excluding goodwill)	1,910,000	2,314,000
Other [Member}
Finite-lived intangible assets [Line Items]
Other amortizable intangible assets	10,979,000	11,442,000
Accumulated amortization	(5,369,000)	(4,223,000)
Intangible assets, net (excluding goodwill)	$ 5,610,000	$ 7,219,000