Basis of presentation and significant accounting policies - Inventory (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Materials and supplies
Impairment expense	$ 0	$ 162,027	$ 2,374,888
Nonrecurring | Level 2
Materials and supplies
Impairment expense	0	963	4,133
Materials and Supplies | Nonrecurring | Level 2
Materials and supplies
Impairment expense	0	963	2,819
Line-fill | Nonrecurring | Level 2
Materials and supplies
Impairment expense	$ 0	$ 0	$ 1,314