Cash, Cash Equivalents, and Investments - Summary of Cash, Cash Equivalents and Held-to-Maturity Investments by Type (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 349,462	$ 81,967
Gross Unrealized Losses	0	(6)
Fair Value	349,462	81,961
Cash and Cash Equivalents	349,462	75,105
Short-term investments	0	6,862
Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost		27,516
Gross Unrealized Losses		(6)
Fair Value		27,510
Cash and Cash Equivalents		20,884
Short-term investments		6,632
Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost		992
Gross Unrealized Losses		0
Fair Value		992
Cash and Cash Equivalents		762
Short-term investments		230
Level 2 | State and municipal obligations
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost		992
Gross Unrealized Losses		0
Fair Value		992
Cash and Cash Equivalents		762
Short-term investments		230
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	144,095	53,459
Gross Unrealized Losses	0	0
Fair Value	144,095	53,459
Cash and Cash Equivalents	144,095	53,459
Short-term investments	0	0
Money market funds | Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	205,367	20,884
Gross Unrealized Losses	0	0
Fair Value	205,367	20,884
Cash and Cash Equivalents	205,367	20,884
Short-term investments	$ 0	0
Corporate bonds | Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost		6,632
Gross Unrealized Losses		(6)
Fair Value		6,626
Cash and Cash Equivalents		0
Short-term investments		$ 6,632