Annual report pursuant to Section 13 and 15(d)

Equity Method Investments - Summary of Consolidated Financial Position And Results of Operations of Significant Entities, Accounted for Using Equity Method (Detail)

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Equity Method Investments - Summary of Consolidated Financial Position And Results of Operations of Significant Entities, Accounted for Using Equity Method (Detail) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule Of Results Related To Equity Accounted Investees [Line Items]      
Current assets $ 125 $ 146  
Total assets 3,177 3,181  
Current liabilities 67 45  
Total liabilities 319 302  
Members' equity 2,858 2,879  
Revenue 284 307 $ 310
Income from continuing operations 31 25 60
Net income 31 25 60
Buckeye Wind Energy Class B Holdings LLC [Member]      
Schedule Of Results Related To Equity Accounted Investees [Line Items]      
Current assets 4 5  
Total assets 287 297  
Current liabilities 1 1  
Total liabilities 8 7  
Members' equity 279 290  
Revenue 9 13 5
Income from continuing operations (5) (6) (9)
Net income (5) (6) (9)
MM Solar Parent LLC [Member]      
Schedule Of Results Related To Equity Accounted Investees [Line Items]      
Current assets 4 3  
Total assets 87 88  
Current liabilities 5 4  
Total liabilities 37 39  
Members' equity 50 49  
Revenue 9 12 13
Income from continuing operations 4 5 6
Net income 4 5 6
Helix Fund Vento I, LLC [Member]      
Schedule Of Results Related To Equity Accounted Investees [Line Items]      
Current assets 1    
Total assets 28 11  
Members' equity 28 11  
Revenue 2    
Income from continuing operations 1    
Net income 1    
Other Investments [Member]      
Schedule Of Results Related To Equity Accounted Investees [Line Items]      
Current assets 116 138  
Total assets 2,775 2,785  
Current liabilities 61 40  
Total liabilities 274 256  
Members' equity 2,501 2,529  
Revenue 264 282 292
Income from continuing operations 31 26 63
Net income $ 31 $ 26 $ 63