Debt (Debt) (Details) - USD ($) $ in Millions |
Sep. 30, 2017 |
Dec. 31, 2016 |
|---|---|---|
| Debt Instrument [Line Items] | ||
| Debt and Capital Lease Obligations | $ 11,650 | $ 12,179 |
| Debt, Current | 369 | 748 |
| Long-term Debt, Excluding Current Maturities | 11,281 | 11,431 |
| Unsecured Debt [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Capital Lease Obligations | 3,415 | 3,412 |
| Loans Payable [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Capital Lease Obligations | 3,152 | 3,165 |
| Corporate Debt Securities [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Capital Lease Obligations | 1,843 | 2,290 |
| Notes Payable, Other Payables [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Capital Lease Obligations | 1,575 | 1,597 |
| Secured Debt [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Capital Lease Obligations | 1,544 | 1,553 |
| Capital Lease Obligations [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Capital Lease Obligations | $ 121 | $ 162 |