Other intangible assets (Details 2) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Finite-Lived Intangible Assets [Line Items]
Intangible assets, net (excluding goodwill)	$ 11,266,000	$ 15,312,000	$ 11,266,000	$ 15,312,000	$ 13,099,000
Amortization of intangible assets	1,000,000	1,000,000	1,800,000	1,800,000
Estimated amortization expense for amortizable intangible assets [Abstract]
2014	3,300,000		3,300,000
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
Thereafter	2,200,000		2,200,000
Customer relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	21,310,000	21,310,000	21,310,000	21,310,000	21,310,000
Intangible assets, accumulated amortization	(14,734,000)	(12,715,000)	(14,734,000)	(12,715,000)	(13,726,000)
Intangible assets, net (excluding goodwill)	6,576,000	8,595,000	6,576,000	8,595,000	7,584,000
Noncompete agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	5,080,000	6,186,000	5,080,000	6,186,000	6,186,000
Intangible assets, accumulated amortization	(3,936,000)	(4,557,000)	(3,936,000)	(4,557,000)	(4,840,000)
Intangible assets, net (excluding goodwill)	1,144,000	1,629,000	1,144,000	1,629,000	1,346,000
Other intangible assets [Member]
Finite-Lived Intangible Assets [Line Items]
Intangible assets, gross	10,921,000	10,979,000	10,921,000	10,979,000	10,995,000
Intangible assets, accumulated amortization	(7,375,000)	(5,891,000)	(7,375,000)	(5,891,000)	(6,826,000)
Intangible assets, net (excluding goodwill)	$ 3,546,000	$ 5,088,000	$ 3,546,000	$ 5,088,000	$ 4,169,000