Fair value measurements (Recurring Fair Value Measurements) (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross		$ 1,332
Derivative liabilities, gross		(14,458)
Derivative assets (liabilities), net		(13,126)
Derivative assets, amount offset		(1,332)
Derivative liabilities, amounts offset		1,332
Derivative liabilities, net		(13,126)
Predecessor
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross			$ 184
Derivative liabilities, gross			(13,553)
Derivative assets (liabilities), net			(13,369)
Derivative assets, amount offset			(184)
Derivative liabilities, amounts offset			184
Derivative liabilities, net			(13,369)
Recurring Fair Value Measurements
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets (liabilities), net		(13,126)
Derivative assets, amount offset	[1]	(1,332)
Derivative liabilities, amounts offset	[1]	1,332
Derivative assets amount offset (liabilities), net	[1]	0
Derivative assets, net		0
Derivative liabilities, net		(13,126)
Recurring Fair Value Measurements | Predecessor
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets (liabilities), net			(13,369)
Derivative assets, amount offset	[1]		(184)
Derivative liabilities, amounts offset	[1]		184
Derivative assets amount offset (liabilities), net	[1]		0
Derivative assets, net			0
Derivative liabilities, net			(13,369)
Recurring Fair Value Measurements | Significant Other Observable Inputs (Level 2)
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross		1,332
Derivative liabilities, gross		(14,163)
Derivative assets (liabilities), net		(12,831)
Recurring Fair Value Measurements | Significant Other Observable Inputs (Level 2) | Predecessor
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross			184
Derivative liabilities, gross			(13,455)
Derivative assets (liabilities), net			(13,271)
Recurring Fair Value Measurements | Significant Unobservable Inputs (Level 3)
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross		0
Derivative liabilities, gross		(295)
Derivative assets (liabilities), net		$ (295)
Recurring Fair Value Measurements | Significant Unobservable Inputs (Level 3) | Predecessor
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis
Derivative assets, gross			0
Derivative liabilities, gross			(98)
Derivative assets (liabilities), net			$ (98)

[1] Amounts represent the impact of master netting agreements that allow us to net settle positive and negative positions with the same counterparty. Positive and negative positions with counterparties are netted on the balance sheet only to the extent that they relate to the same current versus noncurrent classification.