Cash, Cash Equivalents, and Investments - Summary of Cash, Cash Equivalents and Held-to-Maturity Investments by Type (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 61,834	$ 89,300
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(36)	(67)
Fair Value	61,798	89,233
Cash and Cash Equivalents	28,038	40,651
Short-term investments	33,796	48,415
Long-term investments	0	234
Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	32,039	41,228
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(35)	(57)
Fair Value	32,004	41,171
Cash and Cash Equivalents	1,009	7,849
Short-term investments	31,030	33,379
Long-term investments	0	0
Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	2,766	15,270
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(10)
Fair Value	2,765	15,260
Cash and Cash Equivalents	0	0
Short-term investments	2,766	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	2,517	14,477
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(10)
Fair Value	2,516	14,467
Cash and Cash Equivalents	0	0
Short-term investments	2,517	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	249	793
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	249	793
Cash and Cash Equivalents	0	0
Short-term investments	249	793
Long-term investments	0	0
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	27,029	32,802
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	27,029	32,802
Cash and Cash Equivalents	27,029	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	1,009	7,849
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	1,009	7,849
Cash and Cash Equivalents	1,009	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	31,030	33,379
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(35)	(57)
Fair Value	30,995	33,322
Cash and Cash Equivalents	0	0
Short-term investments	31,030	33,379
Long-term investments	$ 0	$ 0