Cash, Cash Equivalents, and Investments - Summary of Cash, Cash Equivalents and Held-to-Maturity Investments by Type (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 311,631	$ 81,967
Gross Unrealized Losses	(3)	(6)
Fair Value	311,628	81,961
Cash and Cash Equivalents	307,507	75,105
Short-term investments	4,124	6,862
Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	261,207	27,516
Gross Unrealized Losses	(3)	(6)
Fair Value	261,204	27,510
Cash and Cash Equivalents	257,679	20,884
Short-term investments	3,528	6,632
Fair Value, Inputs, Level 2 | State and municipal obligations
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	2,587	992
Gross Unrealized Losses	0	0
Fair Value	2,587	992
Cash and Cash Equivalents	1,991	762
Short-term investments	596	230
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	47,837	53,459
Gross Unrealized Losses	0	0
Fair Value	47,837	53,459
Cash and Cash Equivalents	47,837	53,459
Short-term investments	0	0
Money market funds | Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	256,679	20,884
Gross Unrealized Losses	0	0
Fair Value	256,679	20,884
Cash and Cash Equivalents	256,679	20,884
Short-term investments	0	0
Corporate bonds | Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	4,528	6,632
Gross Unrealized Losses	(3)	(6)
Fair Value	4,525	6,626
Cash and Cash Equivalents	1,000	0
Short-term investments	$ 3,528	$ 6,632