Cash, cash equivalents, and investments - Summary of Cash, Cash Equivalents and Held-to-Maturity Investments by Type (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 59,751	$ 63,395
Gross Unrealized Gains	30	16
Gross Unrealized Losses	(12)	(14)
Fair Value	59,769	63,397
Cash and Money Market Funds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	25,023	42,226
Fair Value	25,023	42,226
State and Municipal Obligations [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	17,419	10,807
Gross Unrealized Gains	26	14
Fair Value	17,445	10,821
Corporate Bonds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	14,692	7,743
Gross Unrealized Gains	4	2
Gross Unrealized Losses	(12)	(14)
Fair Value	14,684	7,731
Certificates of Deposit [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	2,617	2,619
Fair Value	$ 2,617	$ 2,619