Note 1 - General and Basis of Presentation (Tables)

6 Months Ended

Jun. 30, 2022

Notes Tables

Schedule of Cash and Cash Equivalents [Table Text Block]

			December
	June 30,		31,		June 30,
	2022		2021		2021
	(Dollars in thousands)
Cash and cash equivalents	$	263,425	$	239,278	$	250,009
Restricted cash and cash equivalents		92,956		104,166		79,868
Total Cash and cash equivalents and restricted cash and cash equivalents	$	356,381	$	343,444	$	329,877

Schedules of Concentration of Risk, by Risk Factor [Table Text Block]

	Three Months Ended June 30,						Six Months Ended June 30,
	2022			2021			2022			2021
Sierra Pacific Power Company and Nevada Power Company		17.7	%		19.5	%		18.6	%		20.3	%
Southern California Public Power Authority (“SCPPA”)		21.3			25.5			21.6			25.2
Kenya Power and Lighting Co. Ltd. ("KPLC")		15.5			17.3			14.8			16.4

Accounts Receivable, Allowance for Credit Loss [Table Text Block]

	Three Months Ended June 30,					Six Months Ended June 30,
	2022		2021			2022		2021
	(Dollars in thousands)					(Dollars in thousands)
Beginning balance of the allowance for expected credit losses	$	90	$	597		$	90	$	597
Change in the provision for expected credit losses for the period		—		(178	)		—		(178	)
Ending balance of the allowance for expected credit losses	$	90	$	419		$	90	$	419

Contract with Customer, Contract Asset, Contract Liability, and Receivable [Table Text Block]

	June 30,			December 31,
	2022			2021
	(Dollars in thousands)
Contract assets (*)	$	13,823		$	9,692
Contract liabilities (*)	$	(12,707	)	$	(9,248	)

Operating Lease, Lease Income [Table Text Block]

	Three Months Ended June 30,				Six Months Ended June 30,
	2022		2021		2022		2021
	(Dollars in thousands)				(Dollars in thousands)
Lease income relating to lease payments from operating leases	$	127,472	$	113,113	$	267,153	$	238,860