Equity (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ (173)	$ 3	$ 111	$ (70)
Other comprehensive income (loss) before reclassifications	8	42	(20)	129
Amounts reclassified from AOCI	7	3	21	0
Total other comprehensive income, net of tax	15	45	1	129
Impact of NEP deconsolidation	0	0	58	0
Adoption of accounting standards updates	(328)		(328)
Less other comprehensive income attributable to noncontrolling interests		0		(11)
Ending balance	(158)	48	(158)	48
Net Unrealized Gains (Losses) on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(67)	(96)	(77)	(100)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from AOCI	7	10	21	24
Total other comprehensive income, net of tax	7	10	21	24
Impact of NEP deconsolidation			3
Adoption of accounting standards updates	(7)		(7)
Less other comprehensive income attributable to noncontrolling interests		1		(9)
Ending balance	(60)	(85)	(60)	(85)
Net Unrealized Gains (Losses) on Available for Sale Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(5)	268	316	225
Other comprehensive income (loss) before reclassifications	(2)	31	(11)	91
Amounts reclassified from AOCI	0	(6)	0	(23)
Total other comprehensive income, net of tax	(2)	25	(11)	68
Impact of NEP deconsolidation			0
Adoption of accounting standards updates	(312)		(312)
Less other comprehensive income attributable to noncontrolling interests		0		0
Ending balance	(7)	293	(7)	293
Defined Benefit Pension and Other Benefits Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(51)	(76)	(39)	(83)
Other comprehensive income (loss) before reclassifications	(1)	0	(4)	7
Amounts reclassified from AOCI	0	(1)	0	(1)
Total other comprehensive income, net of tax	(1)	(1)	(4)	6
Impact of NEP deconsolidation			0
Adoption of accounting standards updates	(9)		(9)
Less other comprehensive income attributable to noncontrolling interests		0		0
Ending balance	(52)	(77)	(52)	(77)
Net Unrealized Gains (Losses) on Foreign Currency Translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(52)	(71)	(69)	(90)
Other comprehensive income (loss) before reclassifications	10	10	(10)	30
Amounts reclassified from AOCI	0	0	0	0
Total other comprehensive income, net of tax	10	10	(10)	30
Impact of NEP deconsolidation			37
Adoption of accounting standards updates	0		0
Less other comprehensive income attributable to noncontrolling interests		(1)		(2)
Ending balance	(42)	(62)	(42)	(62)
Other Comprehensive Income (Loss) Related to Equity Method Investees
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	2	(22)	(20)	(22)
Other comprehensive income (loss) before reclassifications	1	1	5	1
Amounts reclassified from AOCI	0	0	0	0
Total other comprehensive income, net of tax	1	1	5	1
Impact of NEP deconsolidation			18
Adoption of accounting standards updates	0		0
Less other comprehensive income attributable to noncontrolling interests		0		0
Ending balance	$ 3	$ (21)	$ 3	$ (21)