Cash, Cash Equivalents, and Investments - Summary of Cash, Cash Equivalents and Held-to-Maturity Investments by Type (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 71,604	$ 89,300
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(27)	(67)
Fair Value	71,577	89,233
Cash and Cash Equivalents	41,974	40,651
Short-term investments	29,630	48,415
Long-term investments	0	234
Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	41,988	41,228
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(26)	(57)
Fair Value	41,962	41,171
Cash and Cash Equivalents	19,095	7,849
Short-term investments	22,893	33,379
Long-term investments	0	0
Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	6,737	15,270
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(10)
Fair Value	6,736	15,260
Cash and Cash Equivalents	0	0
Short-term investments	6,737	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	6,392	14,477
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(10)
Fair Value	6,391	14,467
Cash and Cash Equivalents	0	0
Short-term investments	6,392	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	345	793
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	345	793
Cash and Cash Equivalents	0	0
Short-term investments	345	793
Long-term investments	0	0
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	22,879	32,802
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	22,879	32,802
Cash and Cash Equivalents	22,879	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	19,095	7,849
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	19,095	7,849
Cash and Cash Equivalents	19,095	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	22,893	33,379
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(26)	(57)
Fair Value	22,867	33,322
Cash and Cash Equivalents	0	0
Short-term investments	22,893	33,379
Long-term investments	$ 0	$ 0