Note 1 - Business and Significant Accounting Policies - Impact of Adoption (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 01, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue										$ 719,267	$ 692,812	$ 662,591
Investment in an unconsolidated company as of January 1, 2018		$ 71,983				$ 34,084				71,983	34,084
Equity in earnings (losses) of investees, net		6,182	$ (117)	$ 388	$ 1,210	(267)	$ 337	$ (428)	$ (1,599)	7,663	(1,957)	(7,735)
Income from continuing operations										110,111	147,109	96,294
Net income attributable to the Company’s stockholders		18,220	10,581	(343)	69,508	64,620	23,960	8,640	35,194	97,966	132,414	88,708
Retained earnings as of the end of the period		422,222				327,255				422,222	327,255
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Equity in earnings (losses) of investees, net										5,615
Income from continuing operations										108,063
Retained earnings as of the end of the period		420,174								420,174
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Net income attributable to the Company’s stockholders										95,918
Electricity [Member]
Revenue		138,320	116,891	122,179	132,489	128,045	110,876	110,896	115,776	509,879	465,593	436,292
Product [Member]
Revenue		49,717	$ 48,439	$ 54,915	$ 48,672	$ 37,862	$ 44,912	$ 67,587	$ 74,122	201,743	224,483	226,299
Other Revenue [Member]
Revenue										7,645	$ 2,736
Accounting Standards Update 2014-09 [Member]
Investment in an unconsolidated company as of January 1, 2018	$ 24,000
Accounting Standards Update 2014-09 [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Equity in earnings (losses) of investees, net										2,048
Income from continuing operations										2,048
Retained earnings as of the end of the period		$ 2,048								2,048
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Net income attributable to the Company’s stockholders										2,048
Accounting Standards Update 2014-09 [Member] | Electricity [Member]
Revenue
Accounting Standards Update 2014-09 [Member] | Product [Member]
Revenue
Accounting Standards Update 2014-09 [Member] | Other Revenue [Member]
Revenue
Accounting Standards Update 2014-09 [Member] | Electricity and Product Revenue [Member]
Revenue										237,648
Accounting Standards Update 2016-02 [Member] | Electricity [Member]
Electricity Revenues accounted under ASC 840, Leases										$ 481,619