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Basis of Presentation and Summary of Significant Accounting Policies - Investment Categories (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | $ 889,653 | $ 179,528
Gross unrealized gains | 1,654 | 1,695
Gross unrealized losses | 179 | 182
Estimated fair value | 891,128 | 181,041
Bank deposits |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 411,166 | 80,095
Gross unrealized gains | 98 | 6
Gross unrealized losses | 17 | 42
Estimated fair value | 411,247 | 80,059
Corporate bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 76,961 | 55,335
Gross unrealized gains | 4 | 0
Gross unrealized losses | 52 | 96
Estimated fair value | 76,913 | 55,239
Commercial paper |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 288,549 | 27,933
Gross unrealized gains | 3 | 0
Gross unrealized losses | 72 | 20
Estimated fair value | 288,480 | 27,913
U.S. Government bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 110,834 | 8,939
Gross unrealized gains | 0 | 0
Gross unrealized losses | 25 | 10
Estimated fair value | 110,809 | 8,929
Agency bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 0 | 4,991
Gross unrealized gains | 0 | 0
Gross unrealized losses | 0 | 1
Estimated fair value | 0 | 4,990
Municipal bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 2,008 | 2,028
Gross unrealized gains | 0 | 0
Gross unrealized losses | 13 | 13
Estimated fair value | 1,995 | 2,015
Corporate equity securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost | 135 | 207
Gross unrealized gains | 1,549 | 1,689
Gross unrealized losses | 0 | 0
Estimated fair value | $ 1,684 | $ 1,896

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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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