Commitments and contingencies (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	9 Months Ended			1 Months Ended	9 Months Ended		1 Months Ended
	Aug. 09, 2012 court_case	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Jun. 07, 2011 Pending Litigation Minimum	Sep. 30, 2014 Letter of Credit	Sep. 30, 2013 Letter of Credit	Jun. 30, 2014 CO2 Compressor Lease	Sep. 30, 2014 CO2 Compressor
Loss Contingencies [Line Items]
Letters of credit outstanding		$ 920		$ 920
Interest paid		74,446	70,735			0	0
Damages sought					5,000
Number of court cases	2
Number of operating leases								2
Operating leases, future minimum payments due									$ 6,500