Debt (Debt) (Details) - USD ($) $ in Millions |
Jun. 30, 2019 |
Dec. 31, 2018 |
|---|---|---|
| Debt Instrument [Line Items] | ||
| Debt and Lease Obligation | $ 10,724 | $ 10,785 |
| Debt, Current | 263 | 637 |
| Long-term Debt, Excluding Current Maturities | 10,461 | 10,148 |
| Unsecured Debt [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Lease Obligation | 2,990 | 3,036 |
| Loans Payable [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Lease Obligation | 2,979 | 2,976 |
| Corporate Debt Securities [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Lease Obligation | 2,402 | 2,400 |
| Notes Payable, Other Payables [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Lease Obligation | 1,229 | 1,264 |
| Secured Debt [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Lease Obligation | 971 | 974 |
| Capital Lease Obligations [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Lease Obligation | 78 | 105 |
| Revolving Credit Facility [Member] | ||
| Debt Instrument [Line Items] | ||
| Debt and Lease Obligation | $ 75 | $ 30 |