Cash, Cash Equivalents, and Investments - Summary of Cash, Cash Equivalents and Held-to-Maturity Investments by Type (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 89,300	$ 118,305
Gross Unrealized Gains	0	11
Gross Unrealized Losses	(67)	(79)
Fair Value	89,233	118,237
Cash and Cash Equivalents	40,651	59,526
Short-term investments	48,415	50,254
Long-term investments	234	8,525
Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	41,228	38,795
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(57)	(70)
Fair Value	41,171	38,725
Cash and Cash Equivalents	7,849	2,389
Short-term investments	33,379	35,677
Long-term investments	0	729
Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	15,270	22,373
Gross Unrealized Gains	0	11
Gross Unrealized Losses	(10)	(9)
Fair Value	15,260	22,375
Cash and Cash Equivalents	0	0
Short-term investments	15,036	14,577
Long-term investments	234	7,796
Level 2 | State and municipal obligations
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	14,477	19,002
Gross Unrealized Gains	0	11
Gross Unrealized Losses	(10)	(9)
Fair Value	14,467	19,004
Cash and Cash Equivalents	0	0
Short-term investments	14,243	12,000
Long-term investments	234	7,002
Level 2 | Certificates of deposit
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	793	3,371
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	793	3,371
Cash and Cash Equivalents	0	0
Short-term investments	793	2,577
Long-term investments	0	794
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	32,802	57,137
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	32,802	57,137
Cash and Cash Equivalents	32,802	57,137
Short-term investments	0	0
Long-term investments	0	0
Money market funds | Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	7,849	2,389
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	7,849	2,389
Cash and Cash Equivalents	7,849	2,389
Short-term investments	0	0
Long-term investments	0	0
Corporate bonds | Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	33,379	36,406
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(57)	(70)
Fair Value	33,322	36,336
Cash and Cash Equivalents	0	0
Short-term investments	33,379	35,677
Long-term investments	$ 0	$ 729