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Financial Instruments - Information on Available-for-sale Investments (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2018 | Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt securities, fair value | $ 10,176 | $ 10,176 | $ 14,908
Publicly traded equity securities, fair value | 378 | 378 |
Publicly traded equity securities, fair value |  |  | 275
Total debt and publicly traded equity securities, fair value | 10,554 | 10,554 |
Total debt and publicly traded equity securities, fair value |  |  | 15,183
Debt securities, amortized cost | 10,300 | 10,300 | 14,976
Publicly traded equity securities, amortized cost |  |  | 265
Total debt and publicly traded equity securities, amortized cost |  |  | 15,241
Debt securities, unrealized gains | 3 | 3 | 11
Equity securities, unrealized gains |  |  | 16
Total debt and publicly traded equity securities, unrealized gains |  |  | 27
Debt securities, unrealized losses | (127) | (127) | (79)
Equity securities, unrealized losses |  |  | (6)
Total debt and publicly traded equity securities, unrealized losses |  |  | (85)
Unrealized net gain | 10 | 60 |
Commercial paper |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt securities, fair value | 30 | 30 | 159
Debt securities, amortized cost | 30 | 30 | 159
Debt securities, unrealized gains | 0 | 0 | 0
Debt securities, unrealized losses | 0 | 0 | 0
Corporate notes and bonds |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt securities, fair value | 6,866 | 6,866 | 9,806
Debt securities, amortized cost | 6,952 | 6,952 | 9,837
Debt securities, unrealized gains | 2 | 2 | 9
Debt securities, unrealized losses | (88) | (88) | (40)
U.S. government and agency securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt securities, fair value | 1,431 | 1,431 | 2,042
Debt securities, amortized cost | 1,448 | 1,448 | 2,059
Debt securities, unrealized gains | 0 | 0 | 0
Debt securities, unrealized losses | (17) | (17) | (17)
Asset-backed securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt securities, fair value | 1,288 | 1,288 | 1,542
Debt securities, amortized cost | 1,300 | 1,300 | 1,548
Debt securities, unrealized gains | 1 | 1 | 1
Debt securities, unrealized losses | (13) | (13) | (7)
Foreign government bonds |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt securities, fair value | 538 | 538 | 733
Debt securities, amortized cost | 546 | 546 | 739
Debt securities, unrealized gains | 0 | 0 | 0
Debt securities, unrealized losses | (8) | (8) | (6)
Mortgage-backed securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Debt securities, fair value | 23 | 23 | 626
Debt securities, amortized cost | 24 | 24 | 634
Debt securities, unrealized gains | 0 | 0 | 1
Debt securities, unrealized losses | $ (1) | $ (1) | $ (9)

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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