Subsequent events - New derivative contracts (Details) - Crude Oil - Put - Subsequent events $ in Millions	Feb. 16, 2016 USD ($) MMBTU $ / MMBTU
Subsequent Event [Line Items]
Deferred premium | $	$ 4.3
January 2017 - December 2017
Subsequent Event [Line Items]
Aggregate volumes (in MMBtu) | MMBTU	8,040,000	[1]
Floor price (in dollars per unit) | $ / MMBTU	2.5	[1]
January 2018 - December 2018
Subsequent Event [Line Items]
Aggregate volumes (in MMBtu) | MMBTU	8,220,000	[1]
Floor price (in dollars per unit) | $ / MMBTU	2.5	[1]

[1] The associated commodity derivatives will be settled based on the Inside FERC index price for West Texas Waha. There are $4.3 million in deferred premiums associated with these contracts.