Cash, Cash Equivalents, and Investments - Summary of Cash, Cash Equivalents and Held-to-Maturity Investments by Type (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 98,983	$ 90,437
Gross Unrealized Gains	19	18
Gross Unrealized Losses	(56)	(34)
Fair Value	98,946	90,421
Cash and Cash Equivalents	40,354	48,367
Short-term investments	46,252	32,774
Long-term investments	12,377	9,296
State and Municipal Obligations
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	20,276	19,145
Gross Unrealized Gains	19	18
Gross Unrealized Losses	(3)	0
Fair Value	20,292	19,163
Cash and Cash Equivalents	480	175
Short-term investments	13,500	15,891
Long-term investments	6,296	3,079
Certificates of Deposit
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	4,355	2,712
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	4,355	2,712
Cash and Cash Equivalents	0	0
Short-term investments	2,714	1,227
Long-term investments	1,641	1,485
Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	37,127	24,320
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(53)	(34)
Fair Value	37,074	24,286
Cash and Cash Equivalents	2,649	3,932
Short-term investments	30,038	15,656
Long-term investments	4,440	4,732
Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	24,631	21,857
Gross Unrealized Gains	19	18
Gross Unrealized Losses	(3)	0
Fair Value	24,647	21,875
Cash and Cash Equivalents	480	175
Short-term investments	16,214	17,118
Long-term investments	7,937	4,564
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	37,225	44,260
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	37,225	44,260
Cash and Cash Equivalents	37,225	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	2,649	3,932
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	2,649	3,932
Cash and Cash Equivalents	2,649	3,932
Short-term investments	0	0
Long-term investments	0	0
Corporate Bonds
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	34,478	20,388
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(53)	(34)
Fair Value	34,425	20,354
Cash and Cash Equivalents	0	0
Short-term investments	30,038	15,656
Long-term investments	$ 4,440	$ 4,732