Quarterly report pursuant to Section 13 or 15(d)

Basis of Presentation and Summary of Significant Accounting Policies - Investment Categories (Details)

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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