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Investments and Fair Value Measurements - Convertible senior notes (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Maturity period for marketable securities        
Maximum contractual maturity period     19 months  
Weighted average contractual maturity period     6 months  
Fair value transfers        
Fair value of assets transferred from Level 2 to Level 1 $ 0 $ 0 $ 0 $ 0
Fair value of liabilities transferred from Level 1 to Level 2 0   0  
Unrealized losses        
Net unrealized losses 0   0  
Available-for-sale securities sold $ 0 $ 0 $ 0 $ 0