Debt (Letter of Credit) (Details) - USD ($) $ in Millions |
Sep. 30, 2017 |
Dec. 31, 2016 |
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|---|---|---|---|---|
| Line of Credit Facility [Line Items] | ||||
| Letters of Credit Outstanding, Amount | $ 950 | $ 991 | ||
| Revolving Credit Facility [Member] | ||||
| Line of Credit Facility [Line Items] | ||||
| Letters of Credit Outstanding, Amount | [1] | 474 | 535 | |
| CDH [Member] | ||||
| Line of Credit Facility [Line Items] | ||||
| Letters of Credit Outstanding, Amount | 246 | 250 | ||
| Various Project Financing Facilities [Member] | ||||
| Line of Credit Facility [Line Items] | ||||
| Letters of Credit Outstanding, Amount | $ 230 | $ 206 | ||
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