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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, 2018 | Dec. 31, 2017
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Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months, Unrealized Losses | $ (8) | $ (22)
Less than 12 Months, Fair Value | 4,669 | 11,149
12 Months or Greater, Unrealized Losses | (64) | (80)
12 Months or Greater, Fair Value | 7,361 | 9,168
Gross Unrealized Losses | (72) | (102)
Total Estimated Fair Value | 12,030 | 20,317
U.S. treasury securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months, Unrealized Losses | (1) | (2)
Less than 12 Months, Fair Value | 1,339 | 821
12 Months or Greater, Unrealized Losses | (14) | (27)
12 Months or Greater, Fair Value | 1,613 | 3,240
Gross Unrealized Losses | (15) | (29)
Total Estimated Fair Value | 2,952 | 4,061
U.S. government agencies securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months, Unrealized Losses | 0 | (1)
Less than 12 Months, Fair Value | 285 | 206
12 Months or Greater, Unrealized Losses | (6) | (7)
12 Months or Greater, Fair Value | 612 | 700
Gross Unrealized Losses | (6) | (8)
Total Estimated Fair Value | 897 | 906
Non-U.S. government securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months, Unrealized Losses | 0 | (1)
Less than 12 Months, Fair Value | 0 | 203
12 Months or Greater, Unrealized Losses | (2) | (3)
12 Months or Greater, Fair Value | 234 | 461
Gross Unrealized Losses | (2) | (4)
Total Estimated Fair Value | 234 | 664
Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months, Unrealized Losses | (6) | (14)
Less than 12 Months, Fair Value | 2,636 | 7,674
12 Months or Greater, Unrealized Losses | (31) | (32)
12 Months or Greater, Fair Value | 3,602 | 3,561
Gross Unrealized Losses | (37) | (46)
Total Estimated Fair Value | 6,238 | 11,235
Residential mortgage and asset-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months, Unrealized Losses | (1) | (4)
Less than 12 Months, Fair Value | 409 | 2,245
12 Months or Greater, Unrealized Losses | (11) | (11)
12 Months or Greater, Fair Value | 1,300 | 1,206
Gross Unrealized Losses | (12) | (15)
Total Estimated Fair Value | $ 1,709 | $ 3,451

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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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