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Accounting Policies - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Jan. 01, 2019 | Jan. 01, 2018 | Dec. 31, 2017
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  |  | $ 4
Operating lease liability | $ 185 |  | $ 185 |  | $ 198 |  |
Operating lease right-of-use asset | 162 |  | 162 |  | 174 |  |
Deferred rent and lease incentives | $ 23 |  | $ 23 |  | $ 24 |  |
Weighted average common shares adjustment related to stock-based compensation (shares) | 1,300,000 | 1,700,000 | 1,200,000 | 1,700,000 |  |  |
Anti-dilutive potential common shares related to stock-based compensation plans (shares) | 0 | 0 | 0 | 0 |  |  |
Maturities of short term investments |  |  | 3 months |  |  |  |
Equity securities |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Available-for-sale securities, Equity securities |  |  |  |  |  |  | $ 1,600
Equity securities | Accounting Standards Update 2016-01 |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  | $ (221) |
Limited partnerships and similar investments | Accounting Standards Update 2016-01 |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  | $ (4) |

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- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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