Note 2 - New Accounting Pronouncements - Impact of Adoption (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	Jan. 01, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Investment in an unconsolidated company		$ 66,551		$ 66,551		$ 34,084
Total consolidated revenues		178,299	$ 179,364	362,322	$ 369,262
Equity in earnings (losses) of investees, net		388	(428)	1,598	(2,027)
Income from continuing operations		2,659	11,846	76,915	51,463
Net income attributable to the Company’s stockholders		(343)	8,640	69,165	43,834
Retained earnings		405,353		405,353		$ 327,255
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Equity in earnings (losses) of investees, net		(3,237)		(1,293)
Income from continuing operations		(966)		74,024
Net income attributable to the Company’s stockholders		(3,968)		66,274
Retained earnings		401,728		401,728
Electricity [Member]
Revenues		122,179	110,896	254,668	226,672
Electricity, Product and Other revenues accounted under ASC 606		122,179	110,896	254,668	226,672
Product [Member]
Revenues		54,915	67,587	103,587	141,709
Electricity, Product and Other revenues accounted under ASC 606		54,915	67,587	103,587	141,709
Other Revenue [Member]
Revenues		1,205	881	4,067	881
Electricity, Product and Other revenues accounted under ASC 606		1,205	$ 881	4,067	$ 881
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		3,625		2,891
Income from continuing operations		3,625		2,891
Net income attributable to the Company’s stockholders		3,625		2,891
Retained earnings		3,625		3,625
Accounting Standards Update 2014-09 [Member] | Electricity [Member]
Revenues
Electricity revenues accounted under ASC 840, Leases		116,914		242,745
Electricity, Product and Other revenues accounted under ASC 606
Accounting Standards Update 2014-09 [Member] | Product [Member]
Revenues
Electricity, Product and Other revenues accounted under ASC 606
Accounting Standards Update 2014-09 [Member] | Electricity and Product Revenue [Member]
Revenues		61,385		119,577
Electricity, Product and Other revenues accounted under ASC 606		$ 61,385		$ 119,577
Accounting Standards Update 2014-09 [Member] | Other Revenue [Member]
Revenues
Electricity, Product and Other revenues accounted under ASC 606