Regulatory Assets and Liabilities Regulatory Assets and Liabilities - by Reportable Segment (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	$ 661	$ 743
Regulatory Liabilities	1,096	843
Brazil SBU
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	0	0
Regulatory Liabilities	1	1
US SBU
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	602	689
Regulatory Liabilities	1,095	842
MCAC SBU
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	59	54
Regulatory Liabilities	$ 0	$ 0