Cash, cash equivalents, and investments - Summary of Cash, Cash Equivalents and Held-to-Maturity Investments by Type (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 74,009	$ 63,395
Gross Unrealized Gains	24	17
Gross Unrealized Losses	(20)	(14)
Fair Value	74,013	63,398
Cash and cash equivalents	38,615	42,271	35,006	36,127
Short-term investments	28,983	9,101
Long-term investments	6,411	12,023
Cash [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	36,392	37,196
Fair Value	36,392	37,196
Cash and cash equivalents	36,392	37,196
Level 1 [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	17,678	12,773
Gross Unrealized Gains	3	3
Gross Unrealized Losses	(20)	(14)
Fair Value	17,661	12,762
Cash and cash equivalents	2,223	5,030
Short-term investments	12,811	1,102
Long-term investments	2,644	6,641
Level 1 [Member] | Money Market Funds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	2,223	5,030
Fair Value	2,223	5,030
Cash and cash equivalents	2,223	5,030
Level 1 [Member] | Corporate Bonds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	15,455	7,743
Gross Unrealized Gains	3	3
Gross Unrealized Losses	(20)	(14)
Fair Value	15,438	7,732
Short-term investments	12,811	1,102
Long-term investments	2,644	6,641
Level 2 [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	19,939	13,426
Gross Unrealized Gains	21	14
Fair Value	19,960	13,440
Cash and cash equivalents		45
Short-term investments	16,172	7,999
Long-term investments	3,767	5,382
Level 2 [Member] | State and Municipal Obligations [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	17,466	10,807
Gross Unrealized Gains	21	14
Fair Value	17,487	10,821
Cash and cash equivalents		45
Short-term investments	14,689	5,626
Long-term investments	2,777	5,136
Level 2 [Member] | Certificates of Deposit [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	2,473	2,619
Fair Value	2,473	2,619
Short-term investments	1,483	2,373
Long-term investments	$ 990	$ 246