Basis of presentation and significant accounting policies - Long-lived assets, materials and supplies and line-fill (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment expense	$ 878	$ 0
Fair Value, Measurements, Nonrecurring | Level 2 | Materials and Supplies
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment expense	800
Fair Value, Measurements, Nonrecurring | Level 2 | Line-fill
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment expense	$ 100