v3.6.0.2
Debt (Corporate Revolving Facility and other Letters of Credit Facilities) (Details) - USD ($)
$ in Millions
 3 Months Ended
Dec. 31, 2016
Mar. 31, 2016
Dec. 31, 2015
Line of Credit Facility [Line Items]      
Letters of Credit Outstanding, Amount $ 991   $ 755
Revolving Credit Facility [Member]      
Line of Credit Facility [Line Items]      
Extension of Line of Credit Revolver   2 years  
Line of Credit Facility, Increase (Decrease), Net   $ 178  
Line of Credit Facility, Maximum Borrowing Capacity   1,678  
Increase in Letter of Credit Sublimit   250  
Total Letter of Credit Sublimit   1,000  
Revolving Credit Facility [Member]      
Line of Credit Facility [Line Items]      
Future line of credit facility maximum borrowing capacity   $ 1,520  
Letters of Credit Outstanding, Amount 535   316
Standby Letters of Credit [Member]      
Line of Credit Facility [Line Items]      
Letters of Credit Outstanding, Amount 250   241
Various Project Financing Facilities [Member]      
Line of Credit Facility [Line Items]      
Letters of Credit Outstanding, Amount 206   $ 198
Amendment No. 4 [Member] | Revolving Credit Facility [Member]      
Line of Credit Facility [Line Items]      
Line of Credit Facility, Increase (Decrease), Net 112    
Line of Credit Facility, Maximum Borrowing Capacity $ 1,790