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Short-term and Long-term Investments, Restricted Deposits (Tables) | 12 Months Ended
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Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract] |
Short-term and long-term investments and restricted deposits by investment type | Short-term and long-term investments and restricted deposits by investment type consist of the following ($ in millions):
| December 31, 2018 |  | December 31, 2017
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value |  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
U.S. Treasury securities and obligations of U.S. government corporations and agencies | $ | 362 |  |  | $ | 1 |  |  | $ | (2 | ) |  | $ | 361 |  |  | $ | 311 |  |  | $ | — |  |  | $ | (2 | ) |  | $ | 309 |
Corporate securities | 3,190 |  |  | 8 |  |  | (52 | ) |  | 3,146 |  |  | 2,208 |  |  | 12 |  |  | (10 | ) |  | 2,210 |
Restricted certificates of deposit | 433 |  |  | — |  |  | — |  |  | 433 |  |  | 4 |  |  | — |  |  | — |  |  | 4 |
Restricted cash equivalents | 8 |  |  | — |  |  | — |  |  | 8 |  |  | 17 |  |  | — |  |  | — |  |  | 17 |
Municipal securities | 2,196 |  |  | 9 |  |  | (18 | ) |  | 2,187 |  |  | 2,085 |  |  | 12 |  |  | (10 | ) |  | 2,087 |
Asset-backed securities | 686 |  |  | 1 |  |  | (4 | ) |  | 683 |  |  | 437 |  |  | 1 |  |  | (1 | ) |  | 437 |
Residential mortgage-backed securities | 452 |  |  | 1 |  |  | (9 | ) |  | 444 |  |  | 337 |  |  | 1 |  |  | (6 | ) |  | 332 |
Commercial mortgage- backed securities | 366 |  |  | 1 |  |  | (6 | ) |  | 361 |  |  | 272 |  |  | 1 |  |  | (2 | ) |  | 271 |
Private equity investments | 387 |  |  | — |  |  | — |  |  | 387 |  |  | 176 |  |  | — |  |  | — |  |  | 176 |
Life insurance contracts | 128 |  |  | — |  |  | — |  |  | 128 |  |  | 135 |  |  | — |  |  | — |  |  | 135 |
Total | $ | 8,208 |  |  | $ | 21 |  |  | $ | (91 | ) |  | $ | 8,138 |  |  | $ | 5,982 |  |  | $ | 27 |  |  | $ | (31 | ) |  | $ | 5,978 |
Schedule of available-for-sale investments in a continuous unrealized loss position | The fair value of available-for-sale debt securities with gross unrealized losses by investment type and length of time that individual securities have been in a continuous unrealized loss position were as follows ($ in millions):
| December 31, 2018 |  | December 31, 2017
| Less Than 12 Months |  | 12 Months or More |  | Less Than 12 Months |  | 12 Months or More
| Unrealized Losses |  | Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses |  | Fair Value
U.S. Treasury securities and obligations of U.S. government corporations and agencies | $ | — |  |  | $ | 59 |  |  | $ | (2 | ) |  | $ | 202 |  |  | $ | (1 | ) |  | $ | 222 |  |  | $ | (1 | ) |  | $ | 79 |
Corporate securities | (27 | ) |  | 1,389 |  |  | (25 | ) |  | 871 |  |  | (6 | ) |  | 1,044 |  |  | (4 | ) |  | 185 |
Municipal securities | (4 | ) |  | 591 |  |  | (14 | ) |  | 806 |  |  | (7 | ) |  | 943 |  |  | (3 | ) |  | 175 |
Asset-backed securities | (2 | ) |  | 318 |  |  | (2 | ) |  | 168 |  |  | (1 | ) |  | 228 |  |  | — |  |  | 28 |
Residential mortgage- backed securities | (1 | ) |  | 61 |  |  | (8 | ) |  | 233 |  |  | (1 | ) |  | 109 |  |  | (5 | ) |  | 171 |
Commercial mortgage- backed securities | (2 | ) |  | 137 |  |  | (4 | ) |  | 140 |  |  | (1 | ) |  | 112 |  |  | (1 | ) |  | 51 |
Total | $ | (36 | ) |  | $ | 2,555 |  |  | $ | (55 | ) |  | $ | 2,420 |  |  | $ | (17 | ) |  | $ | 2,658 |  |  | $ | (14 | ) |  | $ | 689 |
Contractual maturities of short-term and long-term investments and restricted deposits | The contractual maturities of short-term and long-term investments and restricted deposits are as follows ($ in millions):
| December 31, 2018 |  | December 31, 2017
| Investments |  | Restricted Deposits |  | Investments |  | Restricted Deposits
| Amortized Cost |  | Fair Value |  | Amortized Cost |  | Fair Value |  | Amortized Cost |  | Fair Value |  | Amortized Cost |  | Fair Value
One year or less | $ | 647 |  |  | $ | 646 |  |  | $ | 205 |  |  | $ | 205 |  |  | $ | 474 |  |  | $ | 474 |  |  | $ | 48 |  |  | $ | 47 |
One year through five years | 3,026 |  |  | 2,998 |  |  | 351 |  |  | 350 |  |  | 2,424 |  |  | 2,420 |  |  | 88 |  |  | 88 |
Five years through ten years | 2,387 |  |  | 2,362 |  |  | — |  |  | — |  |  | 1,773 |  |  | 1,779 |  |  | — |  |  | — |
Greater than ten years | 88 |  |  | 89 |  |  | — |  |  | — |  |  | 129 |  |  | 130 |  |  | — |  |  | — |
Asset-backed securities | 1,504 |  |  | 1,488 |  |  | — |  |  | — |  |  | 1,046 |  |  | 1,040 |  |  | — |  |  | — |
Total | $ | 7,652 |  |  | $ | 7,583 |  |  | $ | 556 |  |  | $ | 555 |  |  | $ | 5,846 |  |  | $ | 5,843 |  |  | $ | 136 |  |  | $ | 135 |

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- Definition Tabular disclosure of maturities of an entity's investments as well as any other information pertinent to the investments. + References No definition available. + Details
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- Definition For all investments in an unrealized loss position, including those for which other-than-temporary impairments have not been recognized in earnings (including investments for which a portion of an other-than-temporary impairment has been recognized in other comprehensive income), a tabular disclosure of the aggregate related fair value of investments with unrealized losses and the aggregate amount of unrealized losses (that is, the amount by which amortized cost basis exceeds fair value). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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