Income taxes - Income tax (expense) benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current taxes:
Federal	$ 0	$ 0	$ 0
State	(1,800)	0	0
Deferred taxes:
Federal	0	0	152,590
State	0	0	24,355
Total income tax (expense) benefit	$ (1,800)	$ 0	$ 176,945