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Investments - Summary of Investments (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017 | Jun. 06, 2017
--- | --- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | $ 1,824 | $ 2,700 |
Gross Unrealized Gains | 1 | 1 |
Gross Unrealized Losses | 13 | 8 |
Estimated Fair Value | 1,812 | 2,693 |
Corporate debt securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost |  | 101 |
Gross Unrealized Gains |  | 0 |
Gross Unrealized Losses |  | 0 |
Estimated Fair Value |  | 101 |
U.S. treasury notes |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost |  | 68 |
Gross Unrealized Gains |  | 0 |
Gross Unrealized Losses |  | 0 |
Estimated Fair Value |  | 68 |
Subtotal - current restricted investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost |  | 169 |
Gross Unrealized Gains |  | 0 |
Gross Unrealized Losses |  | 0 |
Estimated Fair Value |  | 169 |
Corporate debt securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 1,197 | 1,591 |
Gross Unrealized Gains | 1 | 1 |
Gross Unrealized Losses | 7 | 4 |
Estimated Fair Value | 1,191 | 1,588 |
U.S. treasury notes |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 222 | 389 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 1 | 1 |
Estimated Fair Value | 221 | 388 |
GSEs |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 172 | 255 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 2 | 2 |
Estimated Fair Value | 170 | 253 |
Municipal securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 121 | 142 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 2 | 1 |
Estimated Fair Value | 119 | 141 |
Asset-backed securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 93 | 117 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 1 | 0 |
Estimated Fair Value | 92 | 117 |
Certificate of deposit |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 15 | 37 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 0 | 0 |
Estimated Fair Value | 15 | 37 |
Other |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 4 |  |
Gross Unrealized Gains | 0 |  |
Gross Unrealized Losses | 0 |  |
Estimated Fair Value | $ 4 |  |
Subtotal - current investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost |  | 2,531 |
Gross Unrealized Gains |  | 1 |
Gross Unrealized Losses |  | 8 |
Estimated Fair Value |  | $ 2,524 |
Senior Notes | 4.875% Notes |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Percentage of contractual interest rate | 4.875% |  | 4.875%
Senior Notes | 1.625% Notes |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Percentage of contractual interest rate | 1.625% |  |

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