Note 6 - Variable Interest Entities - Assets and Liabilities for the Company's 2015 Variable Interest Entity (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted cash and cash equivalents		$ 78,693	$ 48,825
Property, plant and equipment, net		1,959,578	1,734,691
Construction-in-process		261,690	293,542
Total assets	[1]	3,121,350	2,623,864	$ 2,461,569
Accounts payable and accrued expenses		116,362	153,796
Long-term debt		1,128,997
Other long-term liabilities		16,087	18,853
Total liabilities		1,667,651	1,321,748
Assets:
Restricted cash and cash equivalents		78,693	48,825
Property, plant and equipment, net		1,959,578	1,734,691
Construction-in-process		261,690	293,542
Total assets	[1]	3,121,350	2,623,864	$ 2,461,569
Liabilities:
Accounts payable and accrued expenses		116,362	153,796
Long-term debt		1,128,997
Other long-term liabilities		16,087	18,853
Total liabilities		1,667,651	1,321,748
Variable Interest Entity, Primary Beneficiary [Member]
Property, plant and equipment, net		1,859,228	1,631,900
Construction-in-process		104,085	142,717
Assets:
Property, plant and equipment, net		1,859,228	1,631,900
Construction-in-process		104,085	142,717
Variable Interest Entity, Primary Beneficiary [Member] | Project Debt [Member]
Restricted cash and cash equivalents		76,019	48,676
Other current assets		213,007	124,322
Property, plant and equipment, net		1,552,408	1,252,623
Construction-in-process		90,812	129,832
Other long-term assets		177,723	63,667
Total assets		2,109,969	1,619,120
Accounts payable and accrued expenses		24,245	24,887
Long-term debt		805,850	658,726
Other long-term liabilities		125,769	93,682
Total liabilities		955,864	777,295
Assets:
Restricted cash and cash equivalents		76,019	48,676
Other current assets		213,007	124,322
Property, plant and equipment, net		1,552,408	1,252,623
Construction-in-process		90,812	129,832
Other long-term assets		177,723	63,667
Total assets		2,109,969	1,619,120
Liabilities:
Accounts payable and accrued expenses		24,245	24,887
Long-term debt		805,850	658,726
Other long-term liabilities		125,769	93,682
Total liabilities		955,864	777,295
Variable Interest Entity, Primary Beneficiary [Member] | Power Purchase Agreements [Member]
Restricted cash and cash equivalents		2,304
Other current assets		9,698	18,010
Property, plant and equipment, net		306,820	379,277
Construction-in-process		13,273	12,885
Other long-term assets		9,104	276
Total assets		341,199	410,448
Accounts payable and accrued expenses		2,651	6,863
Long-term debt
Other long-term liabilities		12,483	6,757
Total liabilities		15,134	13,620
Assets:
Restricted cash and cash equivalents		2,304
Other current assets		9,698	18,010
Property, plant and equipment, net		306,820	379,277
Construction-in-process		13,273	12,885
Other long-term assets		9,104	276
Total assets		341,199	410,448
Liabilities:
Accounts payable and accrued expenses		2,651	6,863
Long-term debt
Other long-term liabilities		12,483	6,757
Total liabilities		$ 15,134	$ 13,620

[1] Electricity segment assets include goodwill in the amount of $20.0 million and $7.6 million as of December 31, 2018 and 2017, respectively. Other segment assets include goodwill in the amount of $0 million and $13.5 million as of December 31, 2018 and 2017, respectively. For further information on goodwill, see Note 9 – Intangible assets and goodwill to the consolidated financial statements.