Note 2 - Summary of Significant Accounting Policies - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2019		Sep. 30, 2018		Sep. 30, 2019		Sep. 30, 2018
Total operating revenue	$ 296		$ 292		$ 797		$ 824
Contract amortization	(18)		(17)		(52)		(52)
Mark-to-market for economic hedging activities	(2)				(9)
Less: Mark-to-market for economic hedging activities	2				9
Less: Lease revenue	(252)		(246)		(675)		(694)
Less: Contract amortization	18		17		52		52
Total revenue from contracts with customers	64		63		183		182
Lease Revenue	252		246		675		694
Conventional Generation [Member]
Total operating revenue	90		87		253		251
Contract amortization	(1)		(1)		(4)		(4)
Mark-to-market for economic hedging activities	0				0
Less: Mark-to-market for economic hedging activities	0				0
Less: Lease revenue	(91)		(88)		(257)		(255)
Less: Contract amortization	1		1		4		4
Total revenue from contracts with customers	0		0		0		0
Lease Revenue	91		88		257		255
Renewables [Member]
Total operating revenue	151		152		392		427
Contract amortization	(16)		(15)		(46)		(46)
Mark-to-market for economic hedging activities	(2)				(9)
Less: Mark-to-market for economic hedging activities	2				9
Less: Lease revenue	(160)		(158)		(416)		(438)
Less: Contract amortization	16		15		46		46
Total revenue from contracts with customers	9		9		31		35
Lease Revenue	160		158		416		438
Thermal [Member]
Total operating revenue	55		53		152		146
Contract amortization	(1)		(1)		(2)		(2)
Mark-to-market for economic hedging activities	0				0
Less: Mark-to-market for economic hedging activities	0				0
Less: Lease revenue	(1)		0		(2)		(1)
Less: Contract amortization	1		1		2		2
Total revenue from contracts with customers	55		54		152		147
Lease Revenue	1		(0)		2		1
Energy Revenue [Member]
Total operating revenue	172	[1]	166	[2]	451	[3]	470	[4]
Less: Lease revenue	(163)		(160)		(422)		(444)
Lease Revenue	163		160		422		444
Energy Revenue [Member] | Conventional Generation [Member]
Total operating revenue	2	[1]	2	[2]	4	[3]	5	[4]
Less: Lease revenue	(2)		(2)		(4)		(5)
Lease Revenue	2		2		4		5
Energy Revenue [Member] | Renewables [Member]
Total operating revenue	167	[1]	164	[2]	441	[3]	463	[4]
Less: Lease revenue	(160)		(158)		(416)		(438)
Lease Revenue	160		158		416		438
Energy Revenue [Member] | Thermal [Member]
Total operating revenue	3	[1]	0	[2]	6	[3]	2	[4]
Less: Lease revenue	(1)		0		(2)		(1)
Lease Revenue	1		0		2		1
Capacity Revenue [Member]
Total operating revenue	133	[1]	132	[2]	379	[3]	377	[4]
Less: Lease revenue	(89)		(86)		(253)		(250)
Lease Revenue	89		86		253		250
Capacity Revenue [Member] | Conventional Generation [Member]
Total operating revenue	89	[1]	86	[2]	253	[3]	250	[4]
Less: Lease revenue	(89)		(86)		(253)		(250)
Lease Revenue	89		86		253		250
Capacity Revenue [Member] | Renewables [Member]
Total operating revenue	0	[1]	0	[2]	0	[3]	0	[4]
Less: Lease revenue	0		0		0		0
Lease Revenue	0		0		0		0
Capacity Revenue [Member] | Thermal [Member]
Total operating revenue	44	[1]	46	[2]	126	[3]	127	[4]
Less: Lease revenue	0		0		0		0
Lease Revenue	0		0		0		0
Product and Service, Other [Member]
Total operating revenue	11		11		28		29
Product and Service, Other [Member] | Conventional Generation [Member]
Total operating revenue	0		0		0		0
Product and Service, Other [Member] | Renewables [Member]
Total operating revenue	2		3		6		10
Product and Service, Other [Member] | Thermal [Member]
Total operating revenue	$ 9		$ 8		$ 22		$ 19

[1] The following amounts of energy and capacity revenue relate to leases and are accounted for under ASC 842: (In millions) Conventional Generation Renewables Thermal Total Energy revenue $ 2 $ 160 $ 1 $ 163 Capacity revenue 90 - - 90 $ 92 $ 160 $ 1 $ 253

[2] The following amounts of energy and capacity revenue relate to leases and are accounted for under ASC 840: (In millions) Conventional Generation Renewables Thermal Total Energy revenue $ 2 $ 158 $ - $ 160 Capacity revenue 86 - - 86 $ 88 $ 158 $ - $ 246

[3] The following amounts of energy and capacity revenue relate to leases and are accounted for under ASC 842: (In millions) Conventional Generation Renewables Thermal Total Energy revenue $ 4 $ 416 $ 2 $ 422 Capacity revenue 254 - - 254 $ 258 $ 416 $ 2 $ 676

[4] The following amounts of energy and capacity revenue relate to leases and are accounted for under ASC 840: (In millions) Conventional Generation Renewables Thermal Total Energy revenue $ 5 $ 438 $ 1 $ 444 Capacity revenue 250 - - 250 $ 255 $ 438 $ 1 $ 694