Credit risk (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) customer partner	Dec. 31, 2015 customer partner	Dec. 31, 2014 customer
Concentration Risk [Line Items]
Cash balances exceeded by balance insured by FDIC | $	$ 40.1
Customers | Total revenues
Concentration Risk [Line Items]
Number of major customers	3
Customers | Total revenues | Customer one and two
Concentration Risk [Line Items]
Number of major customers		2	2
Customers | Total revenues | Customer one
Concentration Risk [Line Items]
Concentration risk (as a percent)	48.50%	37.50%	36.00%
Customers | Total revenues | Customer two
Concentration Risk [Line Items]
Concentration risk (as a percent)	23.00%	20.30%	13.70%
Customers | Total revenues | Customer three
Concentration Risk [Line Items]
Concentration risk (as a percent)	17.00%
Customers | Trade Accounts Receivable
Concentration Risk [Line Items]
Number of major customers		2	3
Customers | Purchased Oil Sales
Concentration Risk [Line Items]
Number of major customers	1	1	1
Concentration risk (as a percent)	100.00%	100.00%	100.00%
Customers | Purchased Oil and Other Products Sales
Concentration Risk [Line Items]
Concentration risk (as a percent)	99.70%	99.60%	97.30%
Credit Concentration Risk | Trade Accounts Receivable | Customer one
Concentration Risk [Line Items]
Concentration risk (as a percent)	45.70%	35.30%	16.40%
Credit Concentration Risk | Trade Accounts Receivable | Customer two
Concentration Risk [Line Items]
Concentration risk (as a percent)	24.70%	23.70%	22.50%
Credit Concentration Risk | Trade Accounts Receivable | Customer three
Concentration Risk [Line Items]
Concentration risk (as a percent)	22.60%	18.50%	13.50%
Credit Concentration Risk | Trade Accounts Receivable | Customer four
Concentration Risk [Line Items]
Concentration risk (as a percent)		10.70%	12.50%
Credit Concentration Risk | Trade Accounts Receivable | Customer five
Concentration Risk [Line Items]
Concentration risk (as a percent)			11.60%
Credit Concentration Risk | Partner one and two | Joint operations accounts receivable
Concentration Risk [Line Items]
Number of joint interest partners | partner	1	2
Credit Concentration Risk | Partner One | Joint operations accounts receivable
Concentration Risk [Line Items]
Concentration risk (as a percent)	19.30%	18.90%
Credit Concentration Risk | Partner Two | Joint operations accounts receivable
Concentration Risk [Line Items]
Concentration risk (as a percent)		17.10%