Cash Equivalents and Marketable Securities - Schedule of Estimated Fair Value of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, amortized cost	$ 11,209	$ 43,834
Cash equivalents, estimated fair value	11,209	43,834
Marketable securities, amortized cost	42,619	15,758
Marketable securities, gross unrealized losses	(32)	(4)
Marketable securities, estimated fair value	42,587	15,754
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, amortized cost	3,959	4,584
Cash equivalents, estimated fair value	3,959	4,584
Reverse repurchase agreements
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, amortized cost	7,250	39,250
Cash equivalents, estimated fair value	7,250	$ 39,250
U.S. government securities
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, amortized cost	40,110
Marketable securities, gross unrealized losses	(30)
Marketable securities, estimated fair value	40,080
U.S. treasury securities
Schedule Of Available For Sale Securities [Line Items]
Marketable securities, amortized cost	2,509
Marketable securities, gross unrealized losses	(2)
Marketable securities, estimated fair value	$ 2,507