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Investments - Summary of Investments (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 1,512 |
Gross Unrealized Gains | 1 |
Gross Unrealized Losses | 5 |
Estimated Fair Value | 1,508 |
Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 949 | $ 1,131
Gross Unrealized Gains | 1 | 0
Gross Unrealized Losses | 3 | 8
Estimated Fair Value | 947 | 1,123
U.S. Treasury notes |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 168 | 181
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 168 | 181
GSEs |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 167 | 164
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 1 | 1
Estimated Fair Value | 166 | 163
Municipal securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 113 | 115
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 1 | 1
Estimated Fair Value | 112 | 114
Asset-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 75 | 83
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 1
Estimated Fair Value | 75 | 82
Mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 23 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | 0 |
Estimated Fair Value | 23 |
Certificate of deposit |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 14 | 14
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 14 | 14
Other |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 3 | 4
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | $ 3 | 4
Subtotal - current investments |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 1,692
Gross Unrealized Gains |  | 0
Gross Unrealized Losses |  | 11
Estimated Fair Value |  | $ 1,681

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