Note 19 - Business Segments - Summarized Financial Information Concerning Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended										12 Months Ended
		Dec. 31, 2019		Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Revenues	[1]											$ 746,044		$ 719,267		$ 692,812
Depreciation and amortization expense												148,761		132,233		115,146
Operating income (loss)		$ 54,472		$ 38,719	$ 46,880	$ 53,725	$ 67,982		$ 25,902	$ 36,633	$ 54,593	193,796		185,110		205,018
Segment assets at period end	[2]	3,250,494	[3]				3,121,350	[4]				3,250,494	[3]	3,121,350	[4]	2,623,864	[4]
Expenditures for long-lived assets												279,986		258,521		259,234
Intersegment Eliminations [Member]
Revenues												84,614		48,817		109,040
Segment Reconciling Items [Member]
Segment assets at period end		81,140					71,983					81,140		71,983		34,084
Electricity Segment [Member]
Revenues												540,333		509,879		465,593
Depreciation and amortization expense												138,426		126,181		109,928
Operating income (loss)												177,192		155,546		157,613
Segment assets at period end	[2]	3,044,909	[3]				2,896,938	[4]				3,044,909	[3]	2,896,938	[4]	2,457,514	[4]
Expenditures for long-lived assets												259,898		219,803		252,581
Electricity Segment [Member] | Intersegment Eliminations [Member]
Revenues												0		0		0
Electricity Segment [Member] | Segment Reconciling Items [Member]
Segment assets at period end		81,140					71,983					81,140		71,983		34,084
Product Segment [Member]
Revenues												191,009		201,743		224,483
Depreciation and amortization expense												5,308		4,311		3,470
Operating income (loss)												23,180		38,083		50,543
Segment assets at period end	[2]	126,018	[3]				156,942	[4]				126,018	[3]	156,942	[4]	115,713	[4]
Expenditures for long-lived assets												9,156		9,993		6,653
Product Segment [Member] | Intersegment Eliminations [Member]
Revenues												84,614		48,817		109,040
Product Segment [Member] | Segment Reconciling Items [Member]
Segment assets at period end		0					0					0		0		0
Other Segments [Member]
Revenues												14,702		7,645		2,736
Depreciation and amortization expense												5,027		1,741		1,748
Operating income (loss)												(6,576)		(8,519)		(3,138)
Segment assets at period end	[2]	79,567	[3]				67,470	[4]				79,567	[3]	67,470	[4]	50,637	[4]
Expenditures for long-lived assets												10,932		28,725		0
Other Segments [Member] | Intersegment Eliminations [Member]
Revenues												0		0		0
Other Segments [Member] | Segment Reconciling Items [Member]
Segment assets at period end		$ 0					$ 0					0		0		0
UNITED STATES
Revenues	[1]											377,956	[5]	328,606	[5]	$ 301,132
UNITED STATES | Electricity Segment [Member]
Revenues	[5]											333,797		305,962
UNITED STATES | Product Segment [Member]
Revenues	[5]											30,562		14,999
UNITED STATES | Other Segments [Member]
Revenues	[5]											13,597		7,645
Non-US [Member]
Revenues	[6]											368,088		390,661
Non-US [Member] | Electricity Segment [Member]
Revenues	[6]											206,536		203,917
Non-US [Member] | Product Segment [Member]
Revenues	[6]											160,447		186,744
Non-US [Member] | Other Segments [Member]
Revenues	[6]											$ 1,105		$ 0

[1] Revenues as reported in the geographic area in which they originate.

[2] Electricity segment assets include goodwill in the amount of $20.1 million, $20.0 million and $7.6 as of December 31, 2019, 2018 and 2017, respectively. No goodwill is included in the Product and Energy Storage and Management Services segment assets as of December 31, 2019 and 2018. Energy Storage and Management Services segment assets as December 31, 2017 include goodwill in the amount of $13.5 million. For further information on goodwill, see Note 9 - Intangible assets and goodwill to the consolidated financial statements.

[3] Including unconsolidated investments 81,140 — — 81,140

[4] Including unconsolidated investments 34,084 — — 34,084

[5] Electricity segment revenues in the United States are all accounted under lease accounting, except for $61.3 million and $26.9 million for the years December 31, 2019 and 2018 that are accounted under ASC 606 starting in 2018. Product and Energy Storage and Management Services segment revenues in the United States are accounted under ASC 606, as further described under Note 1 to the consolidated financial statements.

[6] Electricity segment revenues in foreign countries are all accounted under lease accounting. Product and Energy Storage and Management Services segment revenues in foreign countries are accounted under ASC 606 as further described under Note 1 to the consolidated financial statements.