Cash, Cash Equivalents, and Investments - Summary of Cash, Cash Equivalents and Held-to-Maturity Investments by Type (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 104,584	$ 90,437
Gross Unrealized Gains	33	18
Gross Unrealized Losses	(33)	(34)
Fair Value	104,584	90,421
Cash and Cash Equivalents	45,887	48,367
Short-term investments	49,526	32,774
Long-term investments	9,171	9,296
State and Municipal Obligations
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	18,683	19,145
Gross Unrealized Gains	29	18
Gross Unrealized Losses	(3)	0
Fair Value	18,709	19,163
Cash and Cash Equivalents	60	175
Short-term investments	12,675	15,891
Long-term investments	5,948	3,079
Certificates of Deposit
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	3,858	2,712
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	3,858	2,712
Cash and Cash Equivalents	0	0
Short-term investments	2,816	1,227
Long-term investments	1,042	1,485
Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	36,978	24,320
Gross Unrealized Gains	4	0
Gross Unrealized Losses	(30)	(34)
Fair Value	36,952	24,286
Cash and Cash Equivalents	762	3,932
Short-term investments	34,035	15,656
Long-term investments	2,181	4,732
Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	22,541	21,857
Gross Unrealized Gains	29	18
Gross Unrealized Losses	(3)	0
Fair Value	22,567	21,875
Cash and Cash Equivalents	60	175
Short-term investments	15,491	17,118
Long-term investments	6,990	4,564
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	45,065	44,260
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	45,065	44,260
Cash and Cash Equivalents	45,065	44,260
Short-term investments	0	0
Long-term investments	0	0
Money Market Funds
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	762	3,932
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	762	3,932
Cash and Cash Equivalents	762	3,932
Short-term investments	0	0
Long-term investments	0	0
Corporate Bonds
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	36,216	20,388
Gross Unrealized Gains	4	0
Gross Unrealized Losses	(30)	(34)
Fair Value	36,190	20,354
Cash and Cash Equivalents	0	0
Short-term investments	34,035	15,656
Long-term investments	$ 2,181	$ 4,732