Note 9 - Business Segments - Summarized Financial Information Concerning Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017		Dec. 31, 2017
Total consolidated revenues		$ 178,299		$ 179,364		$ 362,322		$ 369,262
Operating income		36,633		53,152		91,226		112,648
Segment assets at period end		2,994,149	[1]	2,510,633	[1]	2,994,149	[1]	2,510,633	[1]	$ 2,623,864
Including unconsolidated investments		66,551		13,957		66,551		13,957
Operating income		36,633		53,152		91,226		112,648
Intersegment Eliminations [Member]
Total consolidated revenues		11,453		16,565		36,280		32,778
Electricity Segment [Member]
Total consolidated revenues		122,179		110,896		254,668		226,672
Operating income		27,462		38,014		73,874		78,912
Segment assets at period end	[1]	2,805,182		2,273,503		2,805,182		2,273,503
Including unconsolidated investments		66,551		13,957		66,551		13,957
Operating income		27,462		38,014		73,874		78,912
Electricity Segment [Member] | Intersegment Eliminations [Member]
Total consolidated revenues
Product Segment [Member]
Total consolidated revenues		54,915		67,587		103,587		141,709
Operating income		10,761		17,035		20,314		35,633
Segment assets at period end	[1]	125,572		187,015		125,572		187,015
Including unconsolidated investments
Operating income		10,761		17,035		20,314		35,633
Product Segment [Member] | Intersegment Eliminations [Member]
Total consolidated revenues		11,453		16,565		36,280		32,778
Other Segments [Member]
Total consolidated revenues		1,205		881		4,067		881
Operating income		(1,590)		(1,897)		(2,962)		(1,897)
Segment assets at period end	[1]	63,395		50,115		63,395		50,115
Including unconsolidated investments
Operating income		(1,590)		(1,897)		(2,962)		(1,897)
Other Segments [Member] | Intersegment Eliminations [Member]
Total consolidated revenues
UNITED STATES
Total consolidated revenues	[2]	74,371				161,110
UNITED STATES | Electricity Segment [Member]
Total consolidated revenues	[2]	73,139				156,822
UNITED STATES | Product Segment [Member]
Total consolidated revenues	[2]	27				221
UNITED STATES | Other Segments [Member]
Total consolidated revenues	[2]	1,205				4,067
Foreign Countries [Member]
Total consolidated revenues	[3]	103,928				201,212
Foreign Countries [Member] | Electricity Segment [Member]
Total consolidated revenues	[3]	49,040				97,846
Foreign Countries [Member] | Product Segment [Member]
Total consolidated revenues	[3]	54,888				103,366
Foreign Countries [Member] | Other Segments [Member]
Total consolidated revenues	[3]

[1] Electricity segment assets include goodwill in the amount of $26.7 million and $6.6 million as of June 30, 2018 and 2017, respectively. Other segment assets include goodwill in the amount of $13.5 million as of June 30, 2018 and 2017.

[2] Electricity segment revenues in the United States are all accounted under ASC 840, Leases, except for $5.3 million and $11.9 million in the three and six months ended June 30, 2018 that are accounted under ASC 606 starting in 2018. Product and Other segment revenues in the United States are accounted under ASC 606, as further described under Note 2 to the consolidated financial statements.

[3] Electricity segment revenues in foreign countries are all accounted under ASC 840, Leases, and Product revenues in foreign countries are accounted under ASC 606 as further described under Note 2 to the consolidated financial statements.