Note 9 - Business Segments - Summarized Financial Information Concerning Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019		Mar. 31, 2018		Dec. 31, 2018
Revenues		$ 199,038		$ 184,023
Operating income (loss)		53,725		54,593
Segment assets at period end		3,143,214	[1]	2,710,817	[1]	$ 3,121,350
Unconsolidated Investments [Member]
Segment assets at period end		71,885		63,109
Intersegment Eliminations [Member]
Revenues		18,261		24,827
Electricity Segment [Member]
Revenues		142,908		132,489
Operating income (loss)		51,551		46,412
Segment assets at period end	[1]	2,950,444		2,542,154
Electricity Segment [Member] | Unconsolidated Investments [Member]
Segment assets at period end		71,885		63,109
Electricity Segment [Member] | Intersegment Eliminations [Member]
Revenues
Product Segment [Member]
Revenues		52,128		48,672
Operating income (loss)		4,252		9,553
Segment assets at period end	[1]	125,248		114,815
Product Segment [Member] | Unconsolidated Investments [Member]
Segment assets at period end
Product Segment [Member] | Intersegment Eliminations [Member]
Revenues		18,261		24,827
Other Segments [Member]
Revenues		4,002		2,862
Operating income (loss)		(2,078)		(1,372)
Segment assets at period end	[1]	67,522		53,848
Other Segments [Member] | Unconsolidated Investments [Member]
Segment assets at period end
Other Segments [Member] | Intersegment Eliminations [Member]
Revenues
UNITED STATES
Revenues	[2]	106,773		86,739
UNITED STATES | Electricity Segment [Member]
Revenues	[2]	91,528		83,683
UNITED STATES | Product Segment [Member]
Revenues	[2]	11,243		194
UNITED STATES | Other Segments [Member]
Revenues	[2]	4,002		2,862
Foreign Countries [Member]
Revenues	[3]	92,265		97,284
Foreign Countries [Member] | Electricity Segment [Member]
Revenues	[3]	51,380		48,806
Foreign Countries [Member] | Product Segment [Member]
Revenues	[3]	40,885		48,478
Foreign Countries [Member] | Other Segments [Member]
Revenues	[3]

[1] Electricity segment assets include goodwill in the amount of $20.1 million and $7.8 million as of March 31, 2019 and 2018, respectively. Other segment assets include goodwill in the amount of $13.5 million as of March 31, 2018. No goodwill is included in the Other segments assets as of March 31, 2019.

[2] Electricity segment revenues in the United States are all accounted under ASC 842, Leases, except for $17.0 million in the three months ended March 31, 2019 that are accounted under ASC 606. For the three months ended March 31, 2018, Electricity segment revenues in the United States are all accounted under ASC 840, Leases, except for $6.7 million that are accounted under ASC 606.

[3] For the three months ended March 31, 2019, Electricity segment revenues in foreign countries are all accounted under ASC 842, Leases, and Product revenues in foreign countries are accounted under ASC 606. For the three months ended March 31, 2018, Electricity segment revenues in foreign countries are all accounted under ASC 840, Leases, and Product revenues in foreign countries are accounted under ASC 606.