Note 19 - Business Segments - Revenue From Major Customers (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		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
Revenues		$ 190,465	$ 166,480	$ 178,299	$ 184,023	$ 166,365	$ 157,185	$ 179,364	$ 189,898	$ 719,267	[1]	$ 692,812	[1]	$ 662,591	[1]
Southern California Public Power Authority [Member]
Revenues	[2]									$ 109,208		$ 70,100		$ 67,566
Percentage of revenues	[2]									15.20%		10.10%		10.20%
Sierra Pacific Power Company And Nevada Power Company [Member]
Revenues	[2],[3]									$ 116,149		$ 125,424		$ 127,226
Percentage of revenues	[2],[3]									16.10%		18.10%		19.20%
Hyundai [Member]
Revenues	[4]									$ 4,379		$ 28,968		$ 100,856
Percentage of revenues	[4]									0.60%		4.20%		15.20%
Kenya Power and Lighting Co LTD [Member]
Revenues	[2]									$ 119,094		$ 110,243		$ 109,270
Percentage of revenues	[2]									16.60%		15.90%		16.50%

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

[2] Revenues reported in Electricity segment.

[3] Subsidiaries of NV Energy, Inc.

[4] Revenues related to the Sarulla project that are reported in Product segment.