Annual report pursuant to Section 13 and 15(d)

Cash Equivalents and Equity Securities - Components of Cash Equivalents and Equity Securities (Details)

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Cash Equivalents and Equity Securities - Components of Cash Equivalents and Equity Securities (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2019
Dec. 31, 2017
Equity Securities, Available-for-sale [Line Items]      
Cash and cash equivalents, adjusted cost $ 53,039 $ 90,498 $ 31,219
Equity securities, adjusted cost basis 563    
Equity securities, gross unrealized gains 25    
Equity securities, estimated fair value 588    
Adjusted Cost 31,788    
Gross unrealized losses 0    
Estimated Fair Value 31,813    
Money market funds      
Equity Securities, Available-for-sale [Line Items]      
Cash and cash equivalents, adjusted cost 31,225 71,248  
Cash and cash equivalents, estimated fair value $ 31,225 $ 71,248