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Basis of Presentation and Summary of Significant Accounting Policies - Investment Categories (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | --- | --- | ---
Equity Securities, FV-NI [Abstract] |  |  |  |
Estimated fair value | $ 13,233 | $ 9,257 |  |
Debt and Equity Securities, FV-NI [Abstract] |  |  |  |
Amortized cost | 1,319,986 | 600,154 |  |
Gross unrealized gains | 3,725 | 1,226 |  |
Gross unrealized losses | (136) | (163) |  |
Estimated fair value | 1,323,575 | 601,217 |  |
Bank deposits |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 673,988 | 311,066 |  |
Gross unrealized gains | 184 | 26 |  |
Gross unrealized losses | (20) | (29) |  |
Estimated fair value | 674,152 | 311,063 |  |
Corporate bonds |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 74,875 | 53,223 |  |
Gross unrealized gains | 56 | 1 |  |
Gross unrealized losses | (1) | (45) |  |
Estimated fair value | 74,930 | 53,179 |  |
Commercial paper |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 563,585 | 225,731 |  |
Gross unrealized gains | 60 | 8 |  |
Gross unrealized losses | (112) | (76) |  |
Estimated fair value | 563,533 | 225,663 |  |
U.S. Government bonds |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 3,013 | 7,982 |  |
Gross unrealized gains | 0 | 0 |  |
Gross unrealized losses | (3) | (9) |  |
Estimated fair value | 3,010 | 7,973 |  |
Municipal bonds |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 0 | 2,017 |  |
Gross unrealized gains | 0 | 0 |  |
Gross unrealized losses | 0 | (4) |  |
Estimated fair value | 0 | 2,013 |  |
Corporate equity securities(1) |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Amortized cost | 4,525 | 135 | $ 4,525 | $ 135
Gross unrealized gains | 2,915 | 1,191 | 2,915 | 1,191
Gross unrealized losses | 0 | 0 | $ 0 | $ 0
Estimated fair value | 7,440 | 1,326 |  |
Warrants |  |  |  |
Equity Securities, FV-NI [Abstract] |  |  |  |
Amortized cost | 0 | 0 |  |
Gross unrealized gains | 510 | 0 |  |
Gross unrealized losses | 0 | 0 |  |
Estimated fair value | $ 510 | $ 0 |  |

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