Note 2 - New Accounting Pronouncements - Impact of Adoption (Details) - USD ($) $ in Thousands		3 Months Ended
	Jan. 01, 2018	Mar. 31, 2018	Mar. 31, 2017		Dec. 31, 2017	Dec. 31, 2016
Investment in an unconsolidated company		$ 63,109			$ 34,084
Total consolidated revenues		184,023	$ 189,898	[1]
Equity in earnings of investees, net		1,210	(1,599)
Income from continuing operations		74,256	39,617
Net income attributable to the Company’s stockholders		69,508	35,194
Retained earnings		410,758	242,098		$ 327,255	$ 215,352
Electricity [Member]
Total consolidated revenues		132,489	115,776
Product [Member]
Revenues		48,672	74,122
Other Revenue [Member]
Revenues		2,862
Accounting Standards Update 2014-09 [Member]
Investment in an unconsolidated company	$ 24,000
Accounting Standards Update 2014-09 [Member] | Electricity [Member]
Revenues
Electricity Revenues accounted under ASC 840, Leases		125,832
Accounting Standards Update 2014-09 [Member] | Product [Member]
Revenues
Accounting Standards Update 2014-09 [Member] | Electricity and Product Revenue [Member]
Revenues		58,191
Accounting Standards Update 2014-09 [Member] | Other Revenue [Member]
Revenues
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Equity in earnings of investees, net		1,944
Income from continuing operations		74,990
Net income attributable to the Company’s stockholders		70,242
Retained earnings		411,492
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Equity in earnings of investees, net		(734)
Income from continuing operations		(734)
Net income attributable to the Company’s stockholders		(734)
Retained earnings		$ (734)

[1] The amounts related to the Other segment are immaterial.