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Summary of Significant Accounting Policies - Recent Accounting Developments - Guidance Adopted in 2017 (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Increase (decrease) in cash flows from operations | $ 1,387 | $ 2,088 | $ 1,349
Increase (decrease) in cash flows from investing | (1,066) | (792) | (1,528)
Adjustment for adoption of accounting pronouncements | (257) |  |
Decrease in cash flows from financing | 485 | 915 | 432
Retained Earnings |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Adjustment for adoption of accounting pronouncements | (286) |  |
Accounting Standards Update 2018-02 |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Reclassification from AOCI to retained earnings tax effect | $ 13 |  |
Accounting Standards Update 2016-18 |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Increase (decrease) in cash flows from operations |  | (53) | 37
Increase (decrease) in cash flows from investing |  | 32 | (43)
Adoption of ASU 2016-16 | Noncurrent Assets |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Adjustment for adoption of accounting pronouncements |  | 267 |
Adoption of ASU 2016-16 | Retained Earnings |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Adjustment for adoption of accounting pronouncements |  | (267) |
Accounting Standards Update 2016-15 | New Accounting Pronouncement, Early Adoption, Effect |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Increase (decrease) in cash flows from operations |  | $ 121 |
Decrease in cash flows from financing |  |  | $ 121

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- Definition Amount of cash inflow (outflow) from financing activities, including discontinued operations. Financing activity cash flows include obtaining resources from owners and providing them with a return on, and a return of, their investment; borrowing money and repaying amounts borrowed, or settling the obligation; and obtaining and paying for other resources obtained from creditors on long-term credit. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 24 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3521-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 26 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3574-108585 + Details
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