Summary of Unrealized Gains and Losses Related to Cash Equivalents and Investments in Marketable Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 26,903		$ 64,246
Gross Unrealized Gains	19		67
Gross Unrealized Losses	(3)		(1)
Fair Value	26,919	[1]	64,312	[1]
Cash and Cash Equivalents | Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	11		11
Gross Unrealized Gains
Gross Unrealized Losses
Fair Value	11		11
Short-term Investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	26,892		64,235
Gross Unrealized Gains	19		67
Gross Unrealized Losses	(3)		(1)
Fair Value	26,908		64,301
Short-term Investments | Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	12,217		7,259
Gross Unrealized Gains
Gross Unrealized Losses
Fair Value	12,217		7,259
Short-term Investments | Corporate Bond Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	7,645		23,151
Gross Unrealized Gains	14		43
Gross Unrealized Losses			(1)
Fair Value	7,659		23,193
Short-term Investments | U.S. Federal Agencies
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost			27,241
Gross Unrealized Gains			10
Gross Unrealized Losses
Fair Value			27,251
Short-term Investments | Foreign Bonds and Notes
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	5,230		4,682
Gross Unrealized Gains	5		14
Gross Unrealized Losses	(3)
Fair Value	5,232		4,696
Short-term Investments | Municipal Obligations
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	1,800		1,902
Gross Unrealized Gains
Gross Unrealized Losses
Fair Value	$ 1,800		$ 1,902

[1] Interest income receivable included in total investment balance was $0.2 million and $0.4 million at September 30, 2013 and December 31, 2012, respectively.