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Cash Equivalents and Investments (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Cost $ 282,683,000 $ 171,784,000
Gross unrealized gain 8,000 5,000
Gross unrealized loss (5,000) (13,000)
Fair value 282,686,000 171,776,000
Short-term investments 118,955,000 102,556,000
Other-than-temporary impairments on available-for-sale securities 0 0
Money market funds    
Debt Securities, Available-for-sale [Line Items]    
Cost 136,258,000 38,354,000
Gross unrealized gain 0 0
Gross unrealized loss 0 0
Fair value 136,258,000 38,354,000
U.S. treasury securities    
Debt Securities, Available-for-sale [Line Items]    
Cost 48,349,000 80,844,000
Gross unrealized gain 6,000 5,000
Gross unrealized loss 0 (5,000)
Fair value 48,355,000 80,844,000
U.S. government agency obligations    
Debt Securities, Available-for-sale [Line Items]    
Cost 5,993,000 52,586,000
Gross unrealized gain 2,000 0
Gross unrealized loss (5,000) (8,000)
Fair value 5,990,000 52,578,000
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Cost 77,082,000 0
Gross unrealized gain 0 0
Gross unrealized loss 0 0
Fair value 77,082,000 0
Repurchase Agreements    
Debt Securities, Available-for-sale [Line Items]    
Cost 15,001,000 0
Gross unrealized gain 0 0
Gross unrealized loss 0 0
Fair value 15,001,000 0
Cash equivalents    
Debt Securities, Available-for-sale [Line Items]    
Fair value $ 163,731,000 $ 69,220,000