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Debt and Credit Facilities - Senior Unsecured Notes (Narrative) (Details) - USD ($) $ in Millions | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Feb. 29, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017
Senior Notes [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  | $ 3,000
Debt Instrument, Redemption Price, Percentage |  | 100.00% |  |  |
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed |  | 101.00% |  |  |
Interest Expense, Long-term Debt |  | $ 1,000 | $ 1,100 | $ 1,000 |
Senior Unsecured Note Issued September 2017 [Member] | Unsecured Debt [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Repayments of Unsecured Debt |  | 2,250 | 750 |  |
Senior Unsecured Note Issued September 2014 [Member] | Unsecured Debt [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Repayments of Unsecured Debt |  | $ 500 |  |  |
Senior Unsecured Note Issued September 2015 [Member] | Unsecured Debt [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Repayments of Unsecured Debt |  |  | $ 1,000 |  |
Subsequent Event [Member] | Senior Unsecured Note Issued September 2014 [Member] | Unsecured Debt [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Repayments of Unsecured Debt | $ 500 |  |  |  |

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