Cash, Cash Equivalents, and Investments - Summary of Cash, Cash Equivalents and Held-to-Maturity Investments by Type (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 61,843	$ 89,300
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(10)	(67)
Fair Value	61,833	89,233
Cash and Cash Equivalents	43,471	40,651
Short-term investments	18,372	48,415
Long-term investments	0	234
Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	27,049	41,228
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(10)	(57)
Fair Value	27,039	41,171
Cash and Cash Equivalents	9,646	7,849
Short-term investments	17,403	33,379
Long-term investments	0	0
Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	969	15,270
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(10)
Fair Value	969	15,260
Cash and Cash Equivalents	0	0
Short-term investments	969	15,036
Long-term investments	0	234
Fair Value, Inputs, Level 2 | State and municipal obligations
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	969	14,477
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(10)
Fair Value	969	14,467
Cash and Cash Equivalents	0	0
Short-term investments	969	14,243
Long-term investments	0	234
Fair Value, Inputs, Level 2 | Certificates of deposit
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	0	793
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	0	793
Cash and Cash Equivalents	0	0
Short-term investments	0	793
Long-term investments	0	0
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	33,825	32,802
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	33,825	32,802
Cash and Cash Equivalents	33,825	32,802
Short-term investments	0	0
Long-term investments	0	0
Money market funds | Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	9,646	7,849
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	9,646	7,849
Cash and Cash Equivalents	9,646	7,849
Short-term investments	0	0
Long-term investments	0	0
Corporate bonds | Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	17,403	33,379
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(10)	(57)
Fair Value	17,393	33,322
Cash and Cash Equivalents	0	0
Short-term investments	17,403	33,379
Long-term investments	$ 0	$ 0