Fair Value Measurements - Cash Equivalents and Marketable Securities Measured at Fair Value on a Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	$ 21,794	$ 36,679
Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	5,196	7,384
U.S. Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	16,598	29,295
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	76,459	89,062
Cash equivalents and marketable securities, total	98,253	125,741
Fair Value, Measurements, Recurring [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	5,196	7,384
Fair Value, Measurements, Recurring [Member] | U.S. Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	16,598	29,295
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	76,459	89,062
Cash equivalents and marketable securities, total	93,057	118,357
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | U.S. Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	16,598	29,295
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents
Cash equivalents and marketable securities, total	5,196	7,384
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	5,196	7,384
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | U.S. Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents
Cash equivalents and marketable securities, total
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | U.S. Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities