Regulatory Assets and Liabilities Regulatory Assets and Liabilities - by Reportable Segment (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	$ 1,509	$ 918
Regulatory Liabilities	2,114	2,053
Brazil SBU
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	787	260
Regulatory Liabilities	1,347	1,336
US SBU
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	631	550
Regulatory Liabilities	767	717
MCAC SBU (El Salvador)
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	91	108
Regulatory Liabilities	$ 0	$ 0