Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash	$ 4,747
Cash equivalents	158,291
Total cash and cash equivalents	163,038	$ 293,503	$ 64,730	$ 91,551
Marketable securities, amortized cost	376,133
Marketable securities, gross unrealized gains	1
Marketable securities, gross unrealized losses	(47)
Marketable securities, estimated fair value	376,087
Total cash, cash equivalents and marketable securities	539,171
Total cash, cash equivalents and marketable securities	539,125
Amortized Cost
Due within one year, amortized cost	345,678
Due between one and two years, amortized cost	30,455
Marketable securities, amortized cost	376,133
Fair Value
Due within one year, estimated fair value	345,644
Due between one and two years, estimated fair value	30,443
Marketable securities, estimated fair value	376,087
Commercial paper
Cash and Cash Equivalents [Line Items]
Cash equivalents	58,127
Marketable securities, amortized cost	174,769
Marketable securities, gross unrealized gains	1
Marketable securities, gross unrealized losses	(1)
Marketable securities, estimated fair value	174,769
Corporate debt securities
Cash and Cash Equivalents [Line Items]
Cash equivalents	1,624
Marketable securities, amortized cost	47,416
Marketable securities, gross unrealized gains	0
Marketable securities, gross unrealized losses	(30)
Marketable securities, estimated fair value	47,386
Money market funds
Cash and Cash Equivalents [Line Items]
Cash equivalents	97,686
Certificate of deposits
Cash and Cash Equivalents [Line Items]
Cash equivalents	854
Marketable securities, amortized cost	118,775
Marketable securities, gross unrealized gains	0
Marketable securities, gross unrealized losses	0
Marketable securities, estimated fair value	118,775
U.S. Treasuries
Cash and Cash Equivalents [Line Items]
Marketable securities, amortized cost	35,173
Marketable securities, gross unrealized gains	0
Marketable securities, gross unrealized losses	(16)
Marketable securities, estimated fair value	$ 35,157