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Balance Sheet Details - Short-Term Investments (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Short-term investments    
Amortized cost $ 98,605 $ 94,723
Unrealized gains 2 7
Unrealized losses (430) (224)
Estimated fair value 98,177 94,506
U.S. Government bonds    
Short-term investments    
Amortized cost 1,799 1,799
Unrealized losses (16) (17)
Estimated fair value $ 1,783 $ 1,782
U.S. Government bonds | Maximum    
Short-term investments    
Maturity 2 years 2 years
U.S. Government agency bonds    
Short-term investments    
Amortized cost $ 4,185 $ 2,698
Unrealized losses (24) (17)
Estimated fair value $ 4,161 $ 2,681
U.S. Government agency bonds | Maximum    
Short-term investments    
Maturity 2 years 2 years
Bank certificates of deposit    
Short-term investments    
Amortized cost $ 11,800 $ 10,300
Unrealized gains   1
Unrealized losses (8) (3)
Estimated fair value $ 11,792 $ 10,298
Bank certificates of deposit | Maximum    
Short-term investments    
Maturity 1 year 1 year
Commercial paper    
Short-term investments    
Amortized cost $ 16,178 $ 11,598
Unrealized losses (6) (5)
Estimated fair value $ 16,172 $ 11,593
Commercial paper | Maximum    
Short-term investments    
Maturity 1 year 1 year
Corporate notes    
Short-term investments    
Amortized cost $ 46,381 $ 51,532
Unrealized gains 2 6
Unrealized losses (243) (121)
Estimated fair value $ 46,140 $ 51,417
Corporate notes | Maximum    
Short-term investments    
Maturity 3 years 3 years
Asset-backed securities    
Short-term investments    
Amortized cost $ 18,262 $ 16,796
Unrealized losses (133) (61)
Estimated fair value $ 18,129 $ 16,735
Asset-backed securities | Maximum    
Short-term investments    
Maturity 3 years 3 years