Commitments and contingencies (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	6 Months Ended			1 Months Ended	6 Months Ended		1 Months Ended
	Aug. 09, 2012 court_case	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 07, 2011 Pending Litigation Minimum	Jun. 30, 2014 Letter of Credit	Jun. 30, 2013 Letter of Credit	Jun. 30, 2014 CO2 Compressor Lease
Loss Contingencies [Line Items]
Letters of credit outstanding		$ 920		$ 920
Interest paid		56,419	53,199			0	0
Damages sought					5,000
Number of court cases	2
Number of operating leases								2
Operating leases, future minimum payments due								$ 6,711