Basis of Presentation and Summary of Significant Accounting Policies - Investment Categories (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Equity Securities, FV-NI [Abstract]
Estimated fair value	$ 8,314	$ 9,257
Debt and Equity Securities, FV-NI [Abstract]
Amortized cost	876,074	600,154
Gross unrealized gains	1,010	1,226
Gross unrealized losses	(2,701)	(163)
Estimated fair value	874,383	601,217
Bank deposits
Debt Securities, Available-for-sale [Line Items]
Amortized cost	490,828	311,066
Gross unrealized gains	294	26
Gross unrealized losses	(11)	(29)
Estimated fair value	491,111	311,063
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	58,581	53,223
Gross unrealized gains	240	1
Gross unrealized losses	0	(45)
Estimated fair value	58,821	53,179
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized cost	322,160	225,731
Gross unrealized gains	86	8
Gross unrealized losses	(29)	(76)
Estimated fair value	322,217	225,663
U.S. Government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	0	7,982
Gross unrealized gains	0	0
Gross unrealized losses	0	(9)
Estimated fair value	0	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
Debt Securities, Available-for-sale [Line Items]
Amortized cost	4,505	135
Gross unrealized gains	325	1,191
Gross unrealized losses	(2,661)	0
Estimated fair value	2,169	1,326
Warrants
Equity Securities, FV-NI [Abstract]
Amortized cost	0	0
Gross unrealized gains	65	0
Gross unrealized losses	0	0
Estimated fair value	$ 65	$ 0