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Debt and Credit Facility - Senior Unsecured Notes (Narrative) (Details) - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2017
2017 Senior Unsecured Notes [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount |  |  |  | $ 3,000,000,000.0
2017 Senior Unsecured Notes Due in September 2018 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount |  |  |  | 750,000,000
2017 Senior Unsecured Notes Due in September 2019 with Floating Rate [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount |  |  |  | 500,000,000
2017 Senior Unsecured Notes Due in September 2019 with Fixed Rate [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount |  |  |  | $ 1,000,000,000.0
Debt Instrument, Interest Rate, Stated Percentage | 1.85% |  |  | 1.85%
2016 Senior Unsecured Notes |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount |  | $ 5,000,000,000 |  |
Senior Notes [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Redemption Price, Percentage | 100.00% |  |  |
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed | 101.00% |  |  |
Interest Expense, Long-term Debt | $ 1,000,000,000 | 907,000,000 | $ 605,000,000 |
2011 Senior Unsecured Notes |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Repayments of Unsecured Debt |  | $ 700,000,000 |  |

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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + 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(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Percentage price of original principal amount of debt at which debt can be redeemed by the issuer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 3 -Subparagraph e -URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823 + Details
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