Note 2 - New Accounting Pronouncements - Impact of Adoption (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Jan. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Investment in an unconsolidated company		$ 67,739		$ 67,739		$ 34,084
Total consolidated revenues		166,480	$ 157,185	528,802	$ 526,447
Equity in earnings (losses) of investees, net		(117)	337	1,481	(1,690)
Income from continuing operations		10,107	27,559	87,022	79,022
Net income attributable to the Company’s stockholders		10,581	23,960	79,746	67,794
Retained earnings as of the end of the period		410,870		410,870		$ 327,255
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Equity in earnings (losses) of investees, net		(15)		(1,308)
Income from continuing operations		10,209		84,233
Net income attributable to the Company’s stockholders		10,683		76,957
Retained earnings as of the end of the period		410,972		410,972
Electricity [Member]
Revenues		116,891	110,876	371,559	337,548
Product [Member]
Revenues		48,439	44,912	152,026	186,621
Other Revenue [Member]
Revenues		1,150	$ 1,397	5,217	$ 2,278
Accounting Standards Update 2014-09 [Member]
Investment in an unconsolidated company	$ 24,000
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Equity in earnings (losses) of investees, net		(102)		2,789
Income from continuing operations		(102)		2,789
Net income attributable to the Company’s stockholders		(102)		2,789
Retained earnings as of the end of the period		(102)		(102)
Accounting Standards Update 2014-09 [Member] | Electricity [Member]
Revenues
Electricity Revenues accounted under ASC 840, Leases		111,114		353,859
Accounting Standards Update 2014-09 [Member] | Electricity and Product Revenue [Member]
Revenues		$ 55,366		$ 174,943
Accounting Standards Update 2014-09 [Member] | Product [Member]
Revenues
Accounting Standards Update 2014-09 [Member] | Other Revenue [Member]
Revenues