Discontinued Operations (Details) $ in Millions			1 Months Ended		3 Months Ended		7 Months Ended	9 Months Ended		12 Months Ended
	Feb. 24, 2014 USD ($)	Jan. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Nov. 30, 2013 USD ($) project MW	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($) business	Mar. 31, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($) business	Jun. 30, 2015 business	Oct. 30, 2013 MW	Sep. 30, 2013
Discontinued Operations Income Loss
Revenue						$ 0			$ 233
Income from operations of discontinued businesses, before income tax						0			49
Income tax expense					$ 0	0		$ 0	(22)
Income from operations of discontinued businesses, after income tax					0	0		0	27
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of businesses classified as discontinued operations after adoption of new accounting standard | business										0
Impairment expenses								276	304
Net loss from disposal and impairments of discontinued businesses					0	0		0	(56)
Sonel [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent												56.00%
Kribi [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent												56.00%
Dibamba [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent												56.00%
Cameroon [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Impairment expenses							$ 101
Carrying amount						435			435
Fair value less costs to sell						$ 334			$ 334
Saurashtra [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from Divestiture of Businesses	$ 8
Generation Capacity Mega Watts | MW											39
US Wind Projects [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Divestiture Of Ownership In Subsidiary Percent				100.00%
Proceeds from Divestiture of Businesses		$ 27
Generation Capacity Mega Watts | MW				234
Number of wind projects | project				3
Deferred proceeds from divestiture of businesses		$ 3
Cameroon [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Equity Interests Sold | business						3			3
Disposal Group, Including Discontinued Operation, Consideration						$ 200			$ 200
Proceeds from Divestiture of Businesses			$ 156
Letter of Credit Related to Sale Agreement						40			40
Net loss from disposal and impairments of discontinued businesses						(7)
Armenia Mountain [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Generation Capacity Mega Watts | MW				101
Buyer option to purchase ownership percent				100.00%
Buyer option to purchase ownership interest, sales price				$ 75
Noncurrent Asset [Member] | Cameroon [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Disposal Group, Including Discontinued Operation, Non-contingent Consideration, Fair Value, Amount to be Received						$ 44			$ 44
Carrying amount					$ 44			$ 44