Basis of presentation and significant accounting policies - Supplemental cash flow information (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014
Non-cash investing information:
Change in accrued capital expenditures		$ (98,958)	$ 23,945
Change in accrued capital contribution to equity method investee	[1]	34,322	(2,598)
Capitalized asset retirement cost		1,675	4,767
Supplemental cash flow information:
Capitalized interest		$ 227	$ 51

[1] See Notes 14, 15 and 19.a for additional discussion regarding our equity method investee.