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Short-term and Long-term Investments, Restricted Deposits - Schedule by investment Type (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |
Amortized Cost | $ 8,208 | $ 5,982 |
Gross Unrealized Gains | 21 | 27 |
Gross Unrealized Losses | (91) | (31) |
Fair Value | 8,138 |  | $ 5,978
U.S. Treasury securities and obligations of U.S. government corporations and agencies |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |
Amortized Cost | 362 | 311 |
Gross Unrealized Gains | 1 | 0 |
Gross Unrealized Losses | (2) | (2) |
Fair Value | 361 | 309 |
Corporate securities |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |
Amortized Cost | 3,190 | 2,208 |
Gross Unrealized Gains | 8 | 12 |
Gross Unrealized Losses | (52) | (10) |
Fair Value | 3,146 | 2,210 |
Restricted certificates of deposit |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |
Amortized Cost | 433 | 4 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 0 | 0 |
Fair Value | 433 | 4 |
Restricted cash equivalents |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |
Amortized Cost | 8 | 17 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 0 | 0 |
Fair Value | 8 | 17 |
Municipal securities |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |
Amortized Cost | 2,196 | 2,085 |
Gross Unrealized Gains | 9 | 12 |
Gross Unrealized Losses | (18) | (10) |
Fair Value | 2,187 | 2,087 |
Asset-backed securities |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |
Amortized Cost | 686 | 437 |
Gross Unrealized Gains | 1 | 1 |
Gross Unrealized Losses | (4) | (1) |
Fair Value | 683 | 437 |
Residential mortgage-backed securities |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |
Amortized Cost | 452 | 337 |
Gross Unrealized Gains | 1 | 1 |
Gross Unrealized Losses | (9) | (6) |
Fair Value | 444 | 332 |
Commercial mortgage- backed securities |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |
Amortized Cost | 366 | 272 |
Gross Unrealized Gains | 1 | 1 |
Gross Unrealized Losses | (6) | (2) |
Fair Value | 361 | 271 |
Private equity investments |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |
Amortized Cost | 387 | 176 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 0 | 0 |
Fair Value | 387 | 176 |
Life insurance contracts |  |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |  |
Amortized Cost | 128 | 135 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 0 | 0 |
Fair Value | $ 128 | $ 135 |

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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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