Balance Sheet Account Detail - Summary of Cash and Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Cash and cash equivalents, marketable securities, cost	$ 247,716	$ 158,469
Cash and cash equivalents, marketable securities, gross unrealized gains	0	0
Cash and cash equivalents, marketable securities, gross unrealized losses	0	0
Cash and cash equivalents, marketable securities, estimated fair value	247,716	158,469
Cash and cash equivalents	247,468	158,222	$ 171,605	$ 139,741
Marketable security, current	248	247
Marketable securities, noncurrent	0	0
Bank Deposits [Member]
Schedule of Investments [Line Items]
Bank deposits, cost	21,574	23,915
Bank deposits, gross unrealized gains	0	0
Bank deposits, gross unrealized losses	0	0
Bank deposits, estimated fair value	21,574	23,915
Cash and cash equivalents	21,574	23,915
Marketable security, current	0	0
Marketable securities, noncurrent	0	0
Money market funds [Member]
Schedule of Investments [Line Items]
Marketable security, amortized cost	225,894	128,563
Marketable security, gross unrealized gains	0	0
Marketable security, gross unrealized losses	0	0
Marketable securities, fair value	225,894	128,563
Cash and cash equivalents	225,894	128,563
Marketable security, current	0	0
Marketable securities, noncurrent	0	0
Bank CDs [Member]
Schedule of Investments [Line Items]
Marketable security, amortized cost	248	5,991
Marketable security, gross unrealized gains	0	0
Marketable security, gross unrealized losses	0	0
Marketable securities, fair value	248	5,991
Cash and cash equivalents	0	5,744
Marketable security, current	248	247
Marketable securities, noncurrent	$ 0	$ 0