| **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** | Jun. 30, 2017 | Dec. 31, 2016 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Assets**                                                                                                                                                                      |          |          |
| Mortgage loans                                                                                                                                                                  | $ 646         | $ 591         |
| **Liabilities**                                                                                                                                                                 |          |          |
| Short term debt                                                                                                                                                                 | 150           | 0             |
| Long term debt                                                                                                                                                                  | 2,561         | 2,710         |
| Carrying Amount                                                                                                                                                                 |          |          |
| **Assets**                                                                                                                                                                      |          |          |
| Mortgage loans                                                                                                                                                                  | 646           | 591           |
| Note receivable                                                                                                                                                                 | 45            |          |
| **Liabilities**                                                                                                                                                                 |          |          |
| Short term debt                                                                                                                                                                 | 150           |          |
| Long term debt                                                                                                                                                                  | 2,561         | 2,710         |
| Estimated Fair Value                                                                                                                                                            |          |          |
| **Assets**                                                                                                                                                                      |          |          |
| Mortgage loans                                                                                                                                                                  | 655           | 594           |
| Note receivable                                                                                                                                                                 | 45            |          |
| **Liabilities**                                                                                                                                                                 |          |          |
| Short term debt                                                                                                                                                                 | 154           |          |
| Long term debt                                                                                                                                                                  | 2,819         | 2,952         |
| Level 1 | Estimated Fair Value                                                                                                                                                  |          |          |
| **Assets**                                                                                                                                                                      |          |          |
| Mortgage loans                                                                                                                                                                  | 0             | 0             |
| Note receivable                                                                                                                                                                 | 0             |          |
| **Liabilities**                                                                                                                                                                 |          |          |
| Short term debt                                                                                                                                                                 | 0             |          |
| Long term debt                                                                                                                                                                  | 0             | 0             |
| Level 2 | Estimated Fair Value                                                                                                                                                  |          |          |
| **Assets**                                                                                                                                                                      |          |          |
| Mortgage loans                                                                                                                                                                  | 0             | 0             |
| Note receivable                                                                                                                                                                 | 0             |          |
| **Liabilities**                                                                                                                                                                 |          |          |
| Short term debt                                                                                                                                                                 | 154           |          |
| Long term debt                                                                                                                                                                  | 2,819         | 2,952         |
| Level 3 | Estimated Fair Value                                                                                                                                                  |          |          |
| **Assets**                                                                                                                                                                      |          |          |
| Mortgage loans                                                                                                                                                                  | 655           | 594           |
| Note receivable                                                                                                                                                                 | 45            |          |
| **Liabilities**                                                                                                                                                                 |          |          |
| Short term debt                                                                                                                                                                 | 0             |          |
| Long term debt                                                                                                                                                                  | $ 0           | $ 0           |

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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
For an unclassified balance sheet, the amount due from customers or clients for goods or services that have been delivered or sold in the normal course of business, reduced to their estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(3)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(4)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(9)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(5)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910
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| X                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
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| - References
No definition available.
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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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19,20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910
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