Note 8 - Business Segments - Summarized Financial Information Concerning Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2020		Jun. 30, 2019		Jun. 30, 2020		Jun. 30, 2019		Dec. 31, 2019
Revenues		$ 174,900		$ 184,065		$ 367,013		$ 383,103
Operating income (loss)		48,075		46,880		109,132		100,605
Segment assets at period end		3,418,377	[1],[2]	3,193,781	[2],[3]	3,418,377	[1],[2]	3,193,781	[2],[3]	$ 3,250,494
Intersegment Eliminations [Member]
Revenues		50,453		19,167		59,109		37,428
Segment Reconciling Items [Member]
Segment assets at period end		84,414		71,047		84,414		71,047
Electricity Segment [Member]
Revenues		128,685		129,079		271,541		271,987
Operating income (loss)		45,875		43,475		104,505		95,026
Segment assets at period end	[2]	3,150,770	[1]	2,992,891	[3]	3,150,770	[1]	2,992,891	[3]
Electricity Segment [Member] | Intersegment Eliminations [Member]
Revenues		0		0		0		0
Electricity Segment [Member] | Segment Reconciling Items [Member]
Segment assets at period end		84,414		71,047		84,414		71,047
Product Segment [Member]
Revenues		43,701		52,030		91,112		104,158
Operating income (loss)		3,803		5,307		7,675		9,559
Segment assets at period end	[2]	190,474	[1]	128,524	[3]	190,474	[1]	128,524	[3]
Product Segment [Member] | Intersegment Eliminations [Member]
Revenues		50,453		19,167		59,109		37,428
Product Segment [Member] | Segment Reconciling Items [Member]
Segment assets at period end		0		0		0		0
Other Segments [Member]
Revenues		2,514		2,956		4,360		6,958
Operating income (loss)		(1,603)		(1,902)		(3,048)		(3,980)
Segment assets at period end	[2]	77,133	[1]	72,366	[3]	77,133	[1]	72,366	[3]
Other Segments [Member] | Intersegment Eliminations [Member]
Revenues		0		0		0		0
Other Segments [Member] | Segment Reconciling Items [Member]
Segment assets at period end		0		0		0		0
UNITED STATES
Revenues	[4]	85,210		92,419		179,146		199,192
UNITED STATES | Electricity Segment [Member]
Revenues	[4]	80,427		76,931		172,119		168,459
UNITED STATES | Product Segment [Member]
Revenues	[4]	2,269		12,532		2,667		23,775
UNITED STATES | Other Segments [Member]
Revenues	[4]	2,514		2,956		4,360		6,958
Non-US [Member]
Revenues	[5]	89,690		91,646		187,867		183,911
Non-US [Member] | Electricity Segment [Member]
Revenues	[5]	48,258		52,148		99,422		103,528
Non-US [Member] | Product Segment [Member]
Revenues	[5]	41,432		39,498		88,445		80,383
Non-US [Member] | Other Segments [Member]
Revenues	[5]	$ 0		$ 0		$ 0		$ 0

[1] * Including unconsolidated investments 84,414 - - 84,414

[2] Electricity segment assets include goodwill in the amount of $20.1 million and $20.2 million as of June 30, 2020 and 2019, respectively. No goodwill is included in the Product and ESMS segment assets as of June 30, 2020 and 2019.

[3] Including unconsolidated investments 71,047 - - 71,047

[4] Electricity segment revenues in the United States are all accounted under lease accounting except for 15.6 million and $32.4 million in the three and six months ended June 30, 2020, respectively, that are accounted under ASC 606. For the three and six months ended June 30, 2019, Electricity segment revenues in the United States are all accounted under lease accounting except for 12.9 million and 29.9 million, respectively, that are accounted under ASC 606. Product and ESMS segment revenues in the United States are accounted under ASC 606.

[5] Electricity segment revenues in foreign countries are all accounted under lease accounting. Product segment revenues in foreign countries are accounted under ASC 606.