Cash, Cash Equivalents, and Investments - Summary of Cash, Cash Equivalents and Held-to-Maturity Investments by Type (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 328,954	$ 349,462
Gross Unrealized Losses	(56)	0
Fair Value	328,898	349,462
Cash and Cash Equivalents	223,642	349,462
Short-term investments	105,312	0
Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	122,407
Gross Unrealized Losses	(56)
Fair Value	122,351
Cash and Cash Equivalents	17,095
Short-term investments	105,312
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	123,098	144,095
Gross Unrealized Losses	0	0
Fair Value	123,098	144,095
Cash and Cash Equivalents	123,098	144,095
Short-term investments	0	0
Money market funds | Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	83,449	205,367
Gross Unrealized Losses	0	0
Fair Value	83,449	205,367
Cash and Cash Equivalents	83,449	205,367
Short-term investments	$ 0	$ 0