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Investment in Joint Venture (Details) - USD ($)
$ in Thousands
1 Months Ended 3 Months Ended 6 Months Ended
May 31, 2018
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Apr. 30, 2018
Schedule of Equity Method Investments [Line Items]              
Variable interest entity, assets   $ 45,100   $ 45,100   $ 48,300  
Threshold percentage of fair value that is no less than internal rate of return       20.00%      
Fair value of joint venture threshold       40.00%      
Redeemable noncontrolling interest   46,800   $ 46,800   $ 41,800  
Fair value adjustment of redeemable non-controlling interest   $ 300 $ 0 $ 5,000 $ 0    
Guardant Health AMEA, Inc              
Schedule of Equity Method Investments [Line Items]              
Payments to acquire interest in joint venture $ 9,000            
Equity method investment, shares purchased 40,000            
Equity method investment, ownership percentage 50.00%           50.00%
SoftBank | Guardant Health AMEA, Inc              
Schedule of Equity Method Investments [Line Items]              
Equity method investment, shares purchased 40,000            
Equity method investment, ownership percentage 50.00%            
Redeemable noncontrolling interest, redemption value $ 41,000