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Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities at Estimated Fair Value (Details) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
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Available-for-Sale Securities |  |
Amortized Cost | $ 22,662 | $ 29,685
Gross Unrealized Gains | 3 | 4
Gross Unrealized Losses | (104) | (102)
Fair Value | 22,561 | 29,587
Corporate debt securities |  |
Available-for-Sale Securities |  |
Amortized Cost | 12,144 | 14,790
Gross Unrealized Gains | 3 | 3
Gross Unrealized Losses | (55) | (46)
Fair Value | 12,092 | 14,747
U.S. treasury securities |  |
Available-for-Sale Securities |  |
Amortized Cost | 3,412 | 4,090
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (21) | (29)
Fair Value | 3,391 | 4,061
Residential mortgage and asset-backed securities |  |
Available-for-Sale Securities |  |
Amortized Cost | 2,674 | 4,072
Gross Unrealized Gains | 0 | 1
Gross Unrealized Losses | (17) | (15)
Fair Value | 2,657 | 4,058
U.S. government agencies securities |  |
Available-for-Sale Securities |  |
Amortized Cost | 880 | 934
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (8) | (8)
Fair Value | 872 | 926
Certificates of deposit |  |
Available-for-Sale Securities |  |
Amortized Cost | 3,145 | 5,131
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 3,145 | 5,131
Non-U.S. government securities |  |
Available-for-Sale Securities |  |
Amortized Cost | 407 | 668
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (3) | (4)
Fair Value | $ 404 | $ 664

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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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- 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=82897076&loc=d3e27161-111563 + Details
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