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Short-term and Long-term Investments, Restricted Deposits - By Investment Type (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | $ 8,411 | $ 8,208
Gross Unrealized Gains | 85 | 21
Gross Unrealized Losses | (31) | (91)
Fair Value | 8,465 | 8,138
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 | 251 | 362
Gross Unrealized Gains | 1 | 1
Gross Unrealized Losses | (1) | (2)
Fair Value | 251 | 361
Corporate securities |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 3,318 | 3,190
Gross Unrealized Gains | 40 | 8
Gross Unrealized Losses | (16) | (52)
Fair Value | 3,342 | 3,146
Restricted certificates of deposit |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 474 | 433
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 474 | 433
Restricted cash equivalents |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 6 | 8
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 6 | 8
Municipal securities |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 2,260 | 2,196
Gross Unrealized Gains | 33 | 9
Gross Unrealized Losses | (3) | (18)
Fair Value | 2,290 | 2,187
Asset-backed securities |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 728 | 686
Gross Unrealized Gains | 3 | 1
Gross Unrealized Losses | (3) | (4)
Fair Value | 728 | 683
Residential mortgage-backed securities |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 457 | 452
Gross Unrealized Gains | 4 | 1
Gross Unrealized Losses | (6) | (9)
Fair Value | 455 | 444
Commercial mortgage-backed securities |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 378 | 366
Gross Unrealized Gains | 4 | 1
Gross Unrealized Losses | (2) | (6)
Fair Value | 380 | 361
Private equity investments |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 402 | 387
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 402 | 387
Life insurance contracts |  |
Schedule Of Investments And Restricted Deposits By Type [Line Items] |  |
Amortized Cost | 137 | 128
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | $ 137 | $ 128

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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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