Note 1 - General and Basis of Presentation (Tables)

3 Months Ended

Mar. 31, 2021

Notes Tables

Schedule of Cash and Cash Equivalents [Table Text Block]

	March 31,		December 31,		March 31,
	2021		2020		2020
	(Dollars in thousands)
Cash and cash equivalents	$	376,630	$	448,252	$	231,149
Restricted cash and cash equivalents		88,449		88,526		88,627
Total Cash and cash equivalents and restricted cash and cash equivalents	$	465,079	$	536,778	$	319,776

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

	Three Months Ended March 31,
	2021			2020
Sierra Pacific Power Company and Nevada Power Company		21.4	%		19.2	%
Southern California Public Power Authority (“SCPPA”)		24.9			18.7
Kenya Power and Lighting Co. Ltd. ("KPLC")		15.6			15.4

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

	Three Months Ended March 31,
	2021		2020
	(Dollars in thousands)
Beginning balance of the allowance for expected credit losses	$	597	$	755
Change in the provision for expected credit losses for the period		—		24
Ending balance of the allowance for expected credit losses	$	597	$	779

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

	March 31,			December 31,
	2021			2020
	(Dollars in thousands)
Contract assets (*)	$	20,876		$	24,544
Contract liabilities (*)	$	(12,686	)	$	(11,179	)

Operating Lease, Lease Income [Table Text Block]

	Three Months Ended March 31,
	2021		2020
	(Dollars in thousands)
Lease income relating to lease payments from operating leases	$	125,746	$	126,076