Income Taxes - Reconciliation of Effective Income Tax Rate (Details) - USD ($) $ in Millions	1 Months Ended				12 Months Ended
	Nov. 30, 2015	Apr. 30, 2015	Apr. 30, 2014	Jul. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]
Federal statutory income tax rate					35.00%	35.00%	35.00%
Income tax credits					(11.00%)	(10.00%)	(14.00%)
State income tax, net of federal income tax benefit					(1.00%)	1.00%	(9.00%)
Income tax effect of foreign income					(7.00%)	(3.00%)	(6.00%)
Equity income (loss)					(2.00%)	(2.00%)	1.00%
Other, net					(2.00%)	(2.00%)	2.00%
Effective income tax rate					16.00%	23.00%	7.00%
Production Tax Credit Carryforwards [Abstract]
Years Eligible For Renewable Energy Production Tax Credit					10 years
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred state					$ 53	$ (37)	$ 247
Related Party Tax Expense [Abstract]
Income Taxes Receivable From (Payable To) Related Parties Current					286	1,100
NV Energy Transaction [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred state						$ 161
United Kingdom
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount					$ 39		$ 54
Corporate Income Tax Rate Enacted November 2015, Effective April 1, 2017 [Member] | United Kingdom
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Corporate Income Tax Rate, Foreign	20.00%
Corporate Income Tax Rate Enacted November 2015, Effective April 1, 2017 19% [Member] | United Kingdom
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Corporate Income Tax Rate, Foreign	19.00%
Corporate Income Tax Rate Enacted November 2015, Effective April 1, 2017 18% [Member] | United Kingdom
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Corporate Income Tax Rate, Foreign	18.00%
Corporate Income Tax Rate Enacted July 2013, Effective April 1, 2014 [Member] | United Kingdom
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Corporate Income Tax Rate, Foreign			21.00%	23.00%
Corporate Income Tax Rate Enacted July 2013, Effective April 1, 2015 [Member] | United Kingdom
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Corporate Income Tax Rate, Foreign		20.00%