Debt (Debt) (Details) - USD ($) $ in Millions |
Mar. 31, 2016 |
Dec. 31, 2015 |
|---|---|---|
| Debt Instrument [Line Items] | ||
| Debt and Capital Lease Obligations | $ 11,877 | $ 11,937 |
| Debt, Current | 205 | 221 |
| Long-term Debt, Excluding Current Maturities | 11,672 | 11,716 |
| Unsecured Debt [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Capital Lease Obligations | 3,408 | 3,406 |
| Loans Payable [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Capital Lease Obligations | 3,271 | 3,277 |
| Corporate Debt Securities [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Capital Lease Obligations | 1,790 | 1,789 |
| Notes Payable, Other Payables [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Capital Lease Obligations | 1,665 | 1,715 |
| Secured Debt [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Capital Lease Obligations | 1,562 | 1,565 |
| Capital Lease Obligations [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Capital Lease Obligations | $ 181 | $ 185 |