| **Fair Value (Carrying amount and estimated fair value of financial instrument assets and liabilities which are not measured at fair value) (Details) - USD ($) $ in Millions** | Dec. 31, 2016 | Dec. 31, 2015 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financial Assets [Abstract]**                                                                                                                                                 |          |          |
| Mortgage loans                                                                                                                                                                  | $ 591         | $ 678         |
| **Financial Liabilities [Abstract]**                                                                                                                                            |          |          |
| Short term debt                                                                                                                                                                 | 0             | 350           |
| Long term debt                                                                                                                                                                  | 2,710         | 2,210         |
| Carrying Amount [Member]                                                                                                                                                        |          |          |
| **Financial Assets [Abstract]**                                                                                                                                                 |          |          |
| Mortgage loans                                                                                                                                                                  | 591           | 678           |
| **Financial Liabilities [Abstract]**                                                                                                                                            |          |          |
| Short term debt                                                                                                                                                                 |          | 350           |
| Long term debt                                                                                                                                                                  | 2,710         | 2,210         |
| Estimated Fair Value [Member]                                                                                                                                                   |          |          |
| **Financial Assets [Abstract]**                                                                                                                                                 |          |          |
| Mortgage loans                                                                                                                                                                  | 594           | 688           |
| **Financial Liabilities [Abstract]**                                                                                                                                            |          |          |
| Short term debt                                                                                                                                                                 |          | 360           |
| Long term debt                                                                                                                                                                  | 2,952         | 2,433         |
| Level 1 [Member] | Estimated Fair Value [Member]                                                                                                                                |          |          |
| **Financial Assets [Abstract]**                                                                                                                                                 |          |          |
| Mortgage loans                                                                                                                                                                  | 0             | 0             |
| **Financial Liabilities [Abstract]**                                                                                                                                            |          |          |
| Short term debt                                                                                                                                                                 |          | 0             |
| Long term debt                                                                                                                                                                  | 0             | 0             |
| Level 2 [Member] | Estimated Fair Value [Member]                                                                                                                                |          |          |
| **Financial Assets [Abstract]**                                                                                                                                                 |          |          |
| Mortgage loans                                                                                                                                                                  | 0             | 0             |
| **Financial Liabilities [Abstract]**                                                                                                                                            |          |          |
| Short term debt                                                                                                                                                                 |          | 360           |
| Long term debt                                                                                                                                                                  | 2,952         | 2,433         |
| Level 3 [Member] | Estimated Fair Value [Member]                                                                                                                                |          |          |
| **Financial Assets [Abstract]**                                                                                                                                                 |          |          |
| Mortgage loans                                                                                                                                                                  | 594           | 688           |
| **Financial Liabilities [Abstract]**                                                                                                                                            |          |          |
| Short term debt                                                                                                                                                                 |          | 0             |
| Long term debt                                                                                                                                                                  | $ 0           | $ 0           |

| X                                                                                                                                                                                                                                          |
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| - Definition
Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.
+ References
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