Basis of presentation and significant accounting policies - Debt issuance costs and Asset retirement obligations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Debt issuance costs
Capitalization of debt issuance costs			$ 6,759	$ 7,791
Debt issuance costs, net	$ 25,047		25,047		$ 28,500
Accumulated amortization of debt issuance costs	15,900		15,900		19,400
Write-off of debt issuance costs	0	$ 0	0	124
Debt issuance costs, net	19,434		19,434		21,848
Future amortization expense of debt issuance costs
Remaining 2015	1,116		1,116
2016	4,503		4,503
2017	4,575		4,575
2018	4,349		4,349
2019	2,915		2,915
Thereafter	7,589		7,589
Total	25,047		25,047		28,500
Reconciliation of asset retirement obligations liability
Liability at beginning of period			32,198	21,743	21,743
Liabilities added due to acquisitions, drilling, midstream service asset construction and other			1,675		6,370
Accretion expense	599	$ 442	1,771	$ 1,279	1,787
Liabilities settled upon plugging and abandonment			(122)		(450)
Liabilities removed due to sale of property			(2,005)		0
Revision of estimates			0		2,748
Liability at end of period	33,517		33,517		32,198
Adjustments for New Accounting Principle, Early Adoption
Debt issuance costs
Debt issuance costs, net	$ 19,434		19,434		$ 21,848
Senior Notes | January 2019 Notes Issued January 2011
Debt issuance costs
Write-off of debt issuance costs			$ 6,600