Supplemental cash flow information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Non-cash investing activities:
(Decrease) increase in accrued capital expenditures	$ (43,336)	$ 13,674
Capitalized stock-based compensation	2,102	1,980
Capitalized asset retirement costs	130	124
Non-cash financing activities:
Increase in accrued stock repurchases	4,761	0
Other supplemental cash flow information:
Capitalized interest	$ 255	$ 230