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Investments - Summary of Investments (Details) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017 | Jun. 06, 2017
--- | --- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | $ 2,270 | $ 2,700 |
Gross Unrealized Gains | 0 | 1 |
Gross Unrealized Losses | 14 | 8 |
Estimated Fair Value | 2,256 | 2,693 |
Corporate debt securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 55 | 101 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 0 | 0 |
Estimated Fair Value | 55 | 101 |
U.S. treasury notes |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 25 | 68 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 0 | 0 |
Estimated Fair Value | 25 | 68 |
Subtotal - current restricted investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 80 | 169 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 0 | 0 |
Estimated Fair Value | 80 | 169 |
Corporate debt securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 1,390 | 1,591 |
Gross Unrealized Gains | 0 | 1 |
Gross Unrealized Losses | 8 | 4 |
Estimated Fair Value | 1,382 | 1,588 |
U.S. treasury notes |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 331 | 389 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 1 | 1 |
Estimated Fair Value | 330 | 388 |
GSEs |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 209 | 255 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 2 | 2 |
Estimated Fair Value | 207 | 253 |
Municipal securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 138 | 142 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 2 | 1 |
Estimated Fair Value | 136 | 141 |
Asset-backed securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 100 | 117 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 1 | 0 |
Estimated Fair Value | 99 | 117 |
Certificate of deposit |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 19 | 37 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | 0 | 0 |
Estimated Fair Value | 19 | 37 |
Other |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 3 |  |
Gross Unrealized Gains | 0 |  |
Gross Unrealized Losses | 0 |  |
Estimated Fair Value | 3 |  |
Subtotal - current investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 2,190 | 2,531 |
Gross Unrealized Gains | 0 | 1 |
Gross Unrealized Losses | 14 | 8 |
Estimated Fair Value | $ 2,176 | $ 2,524 |
Senior Notes | 4.875% Notes |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Percentage of contractual interest rate | 4.875% |  | 4.875%

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