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Acquisitions Acquistions (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Feb. 18, 2015
Sep. 30, 2017
Sep. 30, 2017
Dec. 31, 2017
Sep. 16, 2016
Aug. 03, 2017
Alto Sertao II [Member]            
Business Acquisition [Line Items]            
Business Acquisition, Percentage of Voting Interests Acquired           100.00%
Business Combination, Contract Purchase Price           $ 181
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities           348
Business Combination, Contingent Consideration, Liability           $ 30
Payments to Acquire Businesses, Net of Cash Acquired   $ 143        
Bauru Solar Complex [Member]            
Business Acquisition [Line Items]            
Payments to Acquire Businesses, Net of Cash Acquired     $ 55      
Other Payments to Acquire Businesses     $ 140      
Project Financing Disbursed       $ 45    
Main street power [Member]            
Business Acquisition [Line Items]            
Business Acquisition, Percentage of Voting Interests Acquired 100.00%          
Business Combination, Consideration Transferred $ 25          
Payments to Acquire Businesses, Gross $ 20          
Distributed energy [Domain]            
Business Acquisition [Line Items]            
Business Combination, Consideration Transferred         $ 43