Parent Company (Guarantees) (Details) - Parent Company - USD ($) $ in Millions |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Debt Instuments and Warrants | ||
| Condensed Financial Statement | ||
| Aggregate balance | $ 24,286 | $ 19,392 |
| Subsidiary Lease Obligations | ||
| Condensed Financial Statement | ||
| Aggregate balance | $ 1,003 | $ 1,082 |