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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 | Sep. 30, 2019 | Dec. 31, 2018
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Available-for-Sale Debt Securities |  |
Less than 12 Months, Unrealized Losses | $ (2) | $ (1)
Less than 12 Months, Fair Value | 3,817 | 2,804
12 Months or Greater, Unrealized Losses | (1) | (52)
12 Months or Greater, Fair Value | 594 | 7,518
Gross Unrealized Losses | (3) | (53)
Total Estimated Fair Value | 4,411 | 10,322
U.S. treasury securities |  |
Available-for-Sale Debt Securities |  |
Less than 12 Months, Unrealized Losses | (1) | 0
Less than 12 Months, Fair Value | 1,339 | 896
12 Months or Greater, Unrealized Losses | 0 | (9)
12 Months or Greater, Fair Value | 96 | 1,383
Gross Unrealized Losses | (1) | (9)
Total Estimated Fair Value | 1,435 | 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 | 902 | 30
12 Months or Greater, Unrealized Losses | 0 | (5)
12 Months or Greater, Fair Value | 30 | 553
Gross Unrealized Losses | 0 | (5)
Total Estimated Fair Value | 932 | 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 | 25 | 86
12 Months or Greater, Unrealized Losses | 0 | (2)
12 Months or Greater, Fair Value | 0 | 192
Gross Unrealized Losses | 0 | (2)
Total Estimated Fair Value | 25 | 278
Corporate debt securities |  |
Available-for-Sale Debt Securities |  |
Less than 12 Months, Unrealized Losses | (1) | (1)
Less than 12 Months, Fair Value | 1,541 | 1,600
12 Months or Greater, Unrealized Losses | 0 | (28)
12 Months or Greater, Fair Value | 297 | 4,204
Gross Unrealized Losses | (1) | (29)
Total Estimated Fair Value | 1,838 | 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 | 10 | 192
12 Months or Greater, Unrealized Losses | (1) | (8)
12 Months or Greater, Fair Value | 171 | 1,186
Gross Unrealized Losses | (1) | (8)
Total Estimated Fair Value | $ 181 | $ 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, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922890-210455 Reference 2: 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=118939692&loc=d3e27337-111563 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=118942371&loc=SL82922954-210456 + 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, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + 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=118939692&loc=d3e27337-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922890-210455 + 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, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + 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=118939692&loc=d3e27337-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922890-210455 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=118942371&loc=SL82922954-210456 + Details
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