Acquisition and divestiture Divestitures (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013
Revenues:
Midstream service revenue	$ 761	$ 4,071
Total revenues from discontinued operations	761	4,071
Cost and expenses:
Midstream service expense, net	(286)	1,163
Depletion, depreciation and amortization	0	627
Total costs and expenses from discontinued operations	(286)	1,790
Income from discontinued operations before income tax	1,047	2,281
Income tax expense	(321)	(765)
Income from discontinued operations	$ 726	$ 1,516