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Balance Sheet Account Details - Investment Categories (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Short-term investments |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost | $ 600,154 | $ 179,528
Gross unrealized gains | 1,226 | 1,695
Gross unrealized losses | (163) | (182)
Estimated fair value | 601,217 | 181,041
Bank deposits |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost | 311,066 | 80,095
Gross unrealized gains | 26 | 6
Gross unrealized losses | (29) | (42)
Estimated fair value | 311,063 | 80,059
Corporate bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost | 53,223 | 55,335
Gross unrealized gains | 1 | 0
Gross unrealized losses | (45) | (96)
Estimated fair value | 53,179 | 55,239
Corporate equity securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost | 135 | 207
Gross unrealized gains | 1,191 | 1,689
Gross unrealized losses | 0 | 0
Estimated fair value | 1,326 | 1,896
Commercial paper |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost | 225,731 | 27,933
Gross unrealized gains | 8 | 0
Gross unrealized losses | (76) | (20)
Estimated fair value | 225,663 | 27,913
Agency bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost |  | 4,991
Gross unrealized gains |  | 0
Gross unrealized losses |  | (1)
Estimated fair value |  | 4,990
U.S. Government bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost | 7,982 | 8,939
Gross unrealized gains | 0 | 0
Gross unrealized losses | (9) | (10)
Estimated fair value | 7,973 | 8,929
Municipal bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Cost | 2,017 | 2,028
Gross unrealized gains | 0 | 0
Gross unrealized losses | (4) | (13)
Estimated fair value | $ 2,013 | $ 2,015

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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