Note 9 - Intangible Assets and Goodwill (Details Textual) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets, Net, Ending Balance	$ 186,220								$ 186,220
Amortization of Intangible Assets, Total									13,300	$ 11,200	$ 6,900
Increase (Decrease) in Intangible Assets, Current									0	127,000	35,600
Finite Lived Intangible Assets, Write off	0				$ 0				0	0	0
Goodwill, Ending Balance	20,140				19,950				20,140	19,950	21,037
Goodwill, Impairment Loss	0	$ 0	$ 0	$ 0	13,464	$ 0	$ 0	$ 0	0	13,464	0
Storage and Energy Management Service [Member]
Goodwill, Impairment Loss										13,500
Other Segments [Member]
Goodwill, Impairment Loss									0	0	$ 0
Power Purchase Agreements and Intangible Assets Related to Storage Activities [Member]
Finite-Lived Intangible Assets, Net, Ending Balance	186,200				199,900				186,200	199,900
Finite-Lived Intangible Assets, Accumulated Amortization	74,100				61,500				74,100	61,500
Intangible Assets Related to Storage Activities [Member]
Finite-Lived Intangible Assets, Net, Ending Balance	30,200				32,200				30,200	32,200
Finite-Lived Intangible Assets, Accumulated Amortization	$ 5,400				$ 3,400				$ 5,400	$ 3,400