Commitments and contingencies (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Sep. 30, 2014
Unrecorded Unconditional Purchase Obligation [Line Items]
Well service expense	$ 0	$ 500,000	$ 0	$ 0
Deferred tax assets, valuation allowance	327,545,000	$ 1,299,000
Drilling Contracts
Unrecorded Unconditional Purchase Obligation [Line Items]
Future commitments	$ 18,400,000