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Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities in a Continuous Unrealized Loss Position Deemed not to be Other-than-Temporarily Impaired (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018
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Available-for-Sale Debt Securities |  |
Less than 12 Months, Unrealized Losses | $ (1) | $ (1)
Less than 12 Months, Fair Value | 4,688 | 2,804
12 Months or Greater, Unrealized Losses | (22) | (52)
12 Months or Greater, Fair Value | 5,290 | 7,518
Gross Unrealized Losses | (23) | (53)
Total Estimated Fair Value | 9,978 | 10,322
U.S. treasury securities |  |
Available-for-Sale Debt Securities |  |
Less than 12 Months, Unrealized Losses | 0 | 0
Less than 12 Months, Fair Value | 2,971 | 896
12 Months or Greater, Unrealized Losses | (4) | (9)
12 Months or Greater, Fair Value | 917 | 1,383
Gross Unrealized Losses | (4) | (9)
Total Estimated Fair Value | 3,888 | 2,279
U.S. government agencies securities |  |
Available-for-Sale Debt Securities |  |
Less than 12 Months, Unrealized Losses | 0 | 0
Less than 12 Months, Fair Value | 483 | 30
12 Months or Greater, Unrealized Losses | (2) | (5)
12 Months or Greater, Fair Value | 458 | 553
Gross Unrealized Losses | (2) | (5)
Total Estimated Fair Value | 941 | 583
Non-U.S. government securities |  |
Available-for-Sale Debt Securities |  |
Less than 12 Months, Unrealized Losses | 0 | 0
Less than 12 Months, Fair Value | 100 | 86
12 Months or Greater, Unrealized Losses | (1) | (2)
12 Months or Greater, Fair Value | 160 | 192
Gross Unrealized Losses | (1) | (2)
Total Estimated Fair Value | 260 | 278
Corporate debt securities |  |
Available-for-Sale Debt Securities |  |
Less than 12 Months, Unrealized Losses | (1) | (1)
Less than 12 Months, Fair Value | 1,049 | 1,600
12 Months or Greater, Unrealized Losses | (11) | (28)
12 Months or Greater, Fair Value | 2,938 | 4,204
Gross Unrealized Losses | (12) | (29)
Total Estimated Fair Value | 3,987 | 5,804
Residential mortgage and asset-backed securities |  |
Available-for-Sale Debt Securities |  |
Less than 12 Months, Unrealized Losses | 0 | 0
Less than 12 Months, Fair Value | 85 | 192
12 Months or Greater, Unrealized Losses | (4) | (8)
12 Months or Greater, Fair Value | 817 | 1,186
Gross Unrealized Losses | (4) | (8)
Total Estimated Fair Value | $ 902 | $ 1,378

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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), in continuous unrealized loss position for more than 12 months. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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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), in continuous unrealized loss position for less than 12 months. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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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), in unrealized loss position. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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