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Cash and Cash Equivalents and Investments (Tables)
 12 Months Ended
Dec. 31, 2018
Cash and Cash Equivalents and Investments [Abstract]  
Schedule of Cash, Cash Equivalents and Short-term Investments
 
As of December 31, 2018
 
Amortized Cost
 
Gross Unrealized Gains
 
Gross Unrealized Losses
 
Estimated Fair Value
 
 
 
(in thousands)
 
 
Cash and cash equivalents
$
80,386

 
$
—

 
$
—

 
$
80,386

Securities maturing within one year:
 
 
 
 
 
 
 
U.S. treasury securities
73,983

 
—

 
(9
)
 
73,974

Corporate debt securities
5,695

 
—

 
(3
)
 
5,692

Total short-term investments
$
79,678

 
$
—

 
$
(12
)
 
$
79,666

Total cash and cash equivalents and investments
$
160,064

 
$
—

 
$
(12
)
 
$
160,052

 
 
As of December 31, 2017
 
Amortized Cost
 
Gross Unrealized Gains
 
Gross Unrealized Losses
 
Estimated Fair Value
 
 
 
(in thousands)
 
 
Cash and cash equivalents
$
61,661

 
$
—

 
$
—

 
$
61,661

Securities maturing within one year:
 
 
 
 
 
 
 
U.S. treasury securities
222,316

 
—

 
(168
)
 
222,148

Corporate debt securities
27,033

 
—

 
(54
)
 
26,979

Total short-term investments
$
249,349

 
$
—

 
$
(222
)
 
$
249,127

Total cash and cash equivalents and investments
$
311,010

 
$
—

 
$
(222
)
 
$
310,788