Note 9 - Intangible Assets and Goodwill (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets, Net, Ending Balance	$ 199,874,000
Amortization of Intangible Assets, Total	11,200,000	$ 6,900,000	$ 4,400,000
Increase (Decrease) in Intangible Assets, Current	127,000,000	35,600,000	33,000,000
Finite Lived Intangible Assets, Write off	0	0	0
Goodwill, Ending Balance	19,950,000	21,037,000	$ 6,650,000
Goodwill, Impairment Loss	13,464,000
Storage and Energy Management Service [Member]
Goodwill, Impairment Loss	13,500,000
Other Segments [Member]
Goodwill, Ending Balance	0	13,500,000
Goodwill, Impairment Loss	0	0
Power Purchase Agreements and Intangible Assets Related to Storage Activities [Member]
Finite-Lived Intangible Assets, Net, Ending Balance	199,900,000	85,400,000
Finite-Lived Intangible Assets, Accumulated Amortization	61,500,000	50,000,000
Intangible Assets Related to Storage Activities [Member]
Finite-Lived Intangible Assets, Net, Ending Balance	32,200,000	33,800,000
Finite-Lived Intangible Assets, Accumulated Amortization	$ 3,400,000	$ 1,700,000