Basis of Presentation and Summary of Significant Accounting Policies - Investment Categories (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 804,206	$ 179,528
Gross unrealized gains	1,524	1,695
Gross unrealized losses	262	182
Estimated fair value	805,468	181,041
Bank deposits
Debt Securities, Available-for-sale [Line Items]
Amortized cost	361,826	80,095
Gross unrealized gains	36	6
Gross unrealized losses	79	42
Estimated fair value	361,783	80,059
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	84,275	55,335
Gross unrealized gains	1	0
Gross unrealized losses	138	96
Estimated fair value	84,138	55,239
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized cost	243,544	27,933
Gross unrealized gains	10	0
Gross unrealized losses	12	20
Estimated fair value	243,542	27,913
U.S. Government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	112,403	8,939
Gross unrealized gains	14	0
Gross unrealized losses	20	10
Estimated fair value	112,397	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	2,023	2,028
Gross unrealized gains	0	0
Gross unrealized losses	13	13
Estimated fair value	2,010	2,015
Corporate equity securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	135	207
Gross unrealized gains	1,463	1,689
Gross unrealized losses	0	0
Estimated fair value	$ 1,598	$ 1,896

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).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount, before tax, of unrealized loss 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).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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