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Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities at Estimated Fair Value (Details) - USD ($) $ in Millions | Sep. 30, 2019 | Dec. 31, 2018
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Available-for-Sale Debt Securities |  |
Amortized Cost | $ 19,052 | $ 24,216
Gross Unrealized Gains | 4 | 1
Gross Unrealized Losses | (3) | (53)
Available-for-sale debt securities | 19,053 | 24,164
U.S. treasury securities |  |
Available-for-Sale Debt Securities |  |
Amortized Cost | 2,860 | 3,978
Gross Unrealized Gains | 1 | 0
Gross Unrealized Losses | (1) | (9)
Available-for-sale debt securities | 2,860 | 3,969
Certificates of deposit |  |
Available-for-Sale Debt Securities |  |
Amortized Cost | 4,152 | 4,361
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Available-for-sale debt securities | 4,152 | 4,361
U.S. government agencies securities |  |
Available-for-Sale Debt Securities |  |
Amortized Cost | 1,145 | 943
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | (5)
Available-for-sale debt securities | 1,145 | 938
Non-U.S. government securities |  |
Available-for-Sale Debt Securities |  |
Amortized Cost | 192 | 307
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | (2)
Available-for-sale debt securities | 192 | 305
Corporate debt securities |  |
Available-for-Sale Debt Securities |  |
Amortized Cost | 10,472 | 13,095
Gross Unrealized Gains | 3 | 1
Gross Unrealized Losses | (1) | (29)
Available-for-sale debt securities | 10,474 | 13,067
Residential mortgage and asset-backed securities |  |
Available-for-Sale Debt Securities |  |
Amortized Cost | 231 | 1,532
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (1) | (8)
Available-for-sale debt securities | $ 230 | $ 1,524

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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=118939692&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=118939692&loc=d3e27161-111563 + Details
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- Definition Amortized cost 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 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 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=118942338&loc=SL82898722-210454 + Details
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