Cash Equivalents and Short-Term Investments - Cash Equivalents and Available-for-Sale Investments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Cash And Short Term Investments [Line Items]
Short-Term Investments - Fair Value	$ 56,678	$ 37,133
Cash Equivalents [Member]
Cash And Short Term Investments [Line Items]
Cash Equivalents - Amortized Cost	190,956	229,128
Unrealized Gains
Unrealized Losses
Cash Equivalents - Fair Value	190,956	229,128
Cash Equivalents [Member] | Money Market Funds [Member]
Cash And Short Term Investments [Line Items]
Cash Equivalents - Amortized Cost	185,068	226,130
Unrealized Gains
Unrealized Losses
Cash Equivalents - Fair Value	185,068	226,130
Cash Equivalents [Member] | Certificates of Deposit [Member]
Cash And Short Term Investments [Line Items]
Cash Equivalents - Amortized Cost	1,097	998
Unrealized Gains
Unrealized Losses
Cash Equivalents - Fair Value	1,097	998
Cash Equivalents [Member] | Municipal Securities [Member]
Cash And Short Term Investments [Line Items]
Cash Equivalents - Amortized Cost	4,791	2,000
Unrealized Gains
Unrealized Losses
Cash Equivalents - Fair Value	4,791	2,000
Short-Term Investments [Member]
Cash And Short Term Investments [Line Items]
Short-Term Investments - Amortized Cost	56,679	37,131
Unrealized Gains	9	6
Unrealized Losses	(10)	(4)
Short-Term Investments - Fair Value	56,678	37,133
Short-Term Investments [Member] | Certificates of Deposit [Member]
Cash And Short Term Investments [Line Items]
Short-Term Investments - Amortized Cost	15,018	9,612
Unrealized Gains	1	1
Unrealized Losses		(1)
Short-Term Investments - Fair Value	15,019	9,612
Short-Term Investments [Member] | Municipal Securities [Member]
Cash And Short Term Investments [Line Items]
Short-Term Investments - Amortized Cost	21,949	19,701
Unrealized Gains	6	4
Unrealized Losses	(4)	(2)
Short-Term Investments - Fair Value	21,951	19,703
Short-Term Investments [Member] | U.S. Government Agency Bonds [Member]
Cash And Short Term Investments [Line Items]
Short-Term Investments - Amortized Cost	2,100	4,500
Unrealized Gains	1	1
Unrealized Losses
Short-Term Investments - Fair Value	2,101	4,501
Short-Term Investments [Member] | Corporate Bonds and Commercial Paper [Member]
Cash And Short Term Investments [Line Items]
Short-Term Investments - Amortized Cost	8,478	3,318
Unrealized Gains
Unrealized Losses	(5)	(1)
Short-Term Investments - Fair Value	8,473	3,317
Short-Term Investments [Member] | International Government Bonds [Member]
Cash And Short Term Investments [Line Items]
Short-Term Investments - Amortized Cost	9,134
Unrealized Gains	1
Unrealized Losses	(1)
Short-Term Investments - Fair Value	$ 9,134