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Summary of Significant Accounting Policies - Short-Term and Long-Term Investment Securities (Details) - USD ($) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Accounting Policies [Abstract] |  |  |
Gains (losses) on securities | $ 0 | $ 0 |
Debt securities, held-to-maturity, sold, realized gain (loss) | 0 | $ 0 |
Available-for-sale securities |  |  |
Available-for-sale investment securities | 867,000,000 |  | $ 1,137,000,000
Total investment securities | 867,000,000 |  | 1,137,000,000
Time deposits |  |  |
Available-for-sale securities |  |  |
Available-for-sale investment securities | 860,000,000 |  | 1,130,000,000
Debt securities |  |  |
Available-for-sale securities |  |  |
Available-for-sale investment securities | $ 7,000,000 |  | $ 7,000,000

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- Definition Amount of 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://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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