Note 9 - Property, Plant and Equipment and Construction-in-process - Activity in Construction and Development (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Balance				$ 248,835					$ 248,835
Write off of unsuccessful exploration costs	$ (303)	$ (1,294)	$ (863)	(557)	$ (1,220)	$ (185)		$ (174)	(3,017)	$ (1,579)	$ (15,439)
Balance	306,709				248,835				306,709	248,835
Projects Under Exploration And Development [Member]
Balance				62,920				73,431	62,920	73,431	69,639
Cost incurred during the year									26,679	11,010	19,231
Write off of unsuccessful exploration costs									(3,017)	(1,579)	(15,439)
Balance	54,447				62,920				54,447	62,920	73,431
Transfer of projects under exploration and development to projects under construction									(32,135)	(19,942)
Projects Under Exploration And Development [Member] | Up-front Bonus Lease Costs [Member]
Balance				26,491				26,618	26,491	26,618	30,141
Cost incurred during the year									1,514	37
Write off of unsuccessful exploration costs									(380)	(164)	(3,523)
Balance	17,385				26,491				17,385	26,491	26,618
Transfer of projects under exploration and development to projects under construction									(10,240)
Projects Under Exploration And Development [Member] | Exploration And Development Costs [Member]
Balance				35,726				45,977	35,726	45,977	38,220
Cost incurred during the year									25,165	10,104	19,231
Write off of unsuccessful exploration costs									(2,637)	(1,415)	(11,474)
Balance	36,359				35,726				36,359	35,726	45,977
Transfer of projects under exploration and development to projects under construction									(21,895)	(18,940)
Projects Under Exploration And Development [Member] | Interest Capitalized [Member]
Balance				703				836	703	836	1,278
Cost incurred during the year										869
Write off of unsuccessful exploration costs											(442)
Balance	703				703				703	703	836
Transfer of projects under exploration and development to projects under construction										(1,002)
Construction in Progress [Member]
Balance				185,915				223,291	185,915	223,291	219,187
Cost incurred during the year									122,757	144,533	135,803
Balance	252,262				185,915				252,262	185,915	223,291
Transfer of projects under exploration and development to projects under construction									32,135	19,942
Transfer of completed projects to property, plant and equipment									(88,545)	(201,851)	(106,096)
Sale of property, plant and equipment											(25,603)
Construction in Progress [Member] | Up-front Bonus Lease Costs [Member]
Balance				27,473				27,473	27,473	27,473	27,473
Cost incurred during the year
Balance	37,713				27,473				37,713	27,473	27,473
Transfer of projects under exploration and development to projects under construction									10,240
Transfer of completed projects to property, plant and equipment
Sale of property, plant and equipment
Construction in Progress [Member] | Drilling And Construction Costs [Member]
Balance				150,467				187,545	150,467	187,545	184,766
Cost incurred during the year									116,247	140,977	132,597
Balance	202,211				150,467				202,211	150,467	187,545
Transfer of projects under exploration and development to projects under construction									21,895	18,940
Transfer of completed projects to property, plant and equipment									(86,398)	(196,995)	(105,126)
Sale of property, plant and equipment											(24,692)
Construction in Progress [Member] | Interest Capitalized [Member]
Balance				$ 7,975				$ 8,273	7,975	8,273	6,948
Cost incurred during the year									6,510	3,556	3,206
Balance	$ 12,338				$ 7,975				12,338	7,975	8,273
Transfer of projects under exploration and development to projects under construction										1,002
Transfer of completed projects to property, plant and equipment									$ (2,147)	(4,856)	(970)
Sale of property, plant and equipment											$ (911)