Cash, Cash Equivalents, and Investments - Summary of Cash, Cash Equivalents and Held-to-Maturity Investments by Type (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 96,805	$ 81,967
Gross Unrealized Losses	(3)	(6)
Fair Value	96,802	81,961
Cash and Cash Equivalents	92,330	75,105
Short-term investments	4,475	6,862
Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	27,256	27,516
Gross Unrealized Losses	(3)	(6)
Fair Value	27,253	27,510
Cash and Cash Equivalents	23,582	20,884
Short-term investments	3,674	6,632
Fair Value, Inputs, Level 2 | State and municipal obligations
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	1,342	992
Gross Unrealized Losses	0	0
Fair Value	1,342	992
Cash and Cash Equivalents	541	762
Short-term investments	801	230
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	68,207	53,459
Gross Unrealized Losses	0	0
Fair Value	68,207	53,459
Cash and Cash Equivalents	68,207	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	22,282	20,884
Gross Unrealized Losses	0	0
Fair Value	22,282	20,884
Cash and Cash Equivalents	22,282	20,884
Short-term investments	0	0
Corporate bonds | Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	4,974	6,632
Gross Unrealized Losses	(3)	(6)
Fair Value	4,971	6,626
Cash and Cash Equivalents	1,300	0
Short-term investments	$ 3,674	$ 6,632