Summary of Significant Accounting Policies - Cash Equivalents and Marketable Securities Measured at Fair Value on a Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	$ 122,334	$ 36,679
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	68,792	89,062
Cash equivalents and marketable securities, total	191,126	125,741
Fair Value, Measurements, Recurring [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	6,915	7,384
Fair Value, Measurements, Recurring [Member] | U.S. Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	115,419	29,295
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	68,792	89,062
Cash equivalents and marketable securities, total	184,211	118,357
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	115,419	29,295
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents and marketable securities, total	6,915	7,384
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities	6,915	7,384
Fair Value, Measurements, Recurring [Member] | 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] | Level 3 [Member] | Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. Treasuries [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities