Equity (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ (178)	$ (40)	$ 111	$ (70)
Other comprehensive income before reclassifications	(2)	40	(28)	88
Amounts reclassified from AOCI	7	4	14	(3)
Total other comprehensive income (loss), net of tax	5	44	(14)	85
Impact of NEP deconsolidation	0	0	58	0
Adoption of accounting standards updates					$ (328)
Less other comprehensive income attributable to noncontrolling interests		(1)		(12)
Ending balance	(173)	3	(173)	3
Net Unrealized Gains (Losses) on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(74)	(101)	(77)	(100)
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from AOCI	7	5	14	14
Total other comprehensive income (loss), net of tax	7	5	14	14
Impact of NEP deconsolidation			3
Adoption of accounting standards updates					(7)
Less other comprehensive income attributable to noncontrolling interests		0		(10)
Ending balance	(67)	(96)	(67)	(96)
Net Unrealized Gains (Losses) on Available for Sale Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(2)	243	316	225
Other comprehensive income before reclassifications	(3)	26	(9)	60
Amounts reclassified from AOCI	0	(1)	0	(17)
Total other comprehensive income (loss), net of tax	(3)	25	(9)	43
Impact of NEP deconsolidation			0
Adoption of accounting standards updates					(312)
Less other comprehensive income attributable to noncontrolling interests		0		0
Ending balance	(5)	268	(5)	268
Defined Benefit Pension and Other Benefits Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(50)	(86)	(39)	(83)
Other comprehensive income before reclassifications	(1)	10	(3)	7
Amounts reclassified from AOCI	0	0	0	0
Total other comprehensive income (loss), net of tax	(1)	10	(3)	7
Impact of NEP deconsolidation			0
Adoption of accounting standards updates					(9)
Less other comprehensive income attributable to noncontrolling interests		0		0
Ending balance	(51)	(76)	(51)	(76)
Net Unrealized Gains (Losses) on Foreign Currency Translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(52)	(75)	(69)	(90)
Other comprehensive income before reclassifications	0	5	(20)	21
Amounts reclassified from AOCI	0	0	0	0
Total other comprehensive income (loss), net of tax	0	5	(20)	21
Impact of NEP deconsolidation			37
Adoption of accounting standards updates					0
Less other comprehensive income attributable to noncontrolling interests		(1)		(2)
Ending balance	(52)	(71)	(52)	(71)
Other Comprehensive Income (Loss) Related to Equity Method Investees
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0	(21)	(20)	(22)
Other comprehensive income before reclassifications	2	(1)	4	0
Amounts reclassified from AOCI	0	0	0	0
Total other comprehensive income (loss), net of tax	2	(1)	4	0
Impact of NEP deconsolidation			18
Adoption of accounting standards updates					$ 0
Less other comprehensive income attributable to noncontrolling interests		0		0
Ending balance	$ 2	$ (22)	$ 2	$ (22)