Fair Value - Fair Value Measurement of Financial Assets (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments	$ 0	$ 0	$ 114,806
Financial assets fair value disclosure	216,835		299,194
Money Market Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	204,838		169,263
Restricted cash	3,374		6,506
Other assets	8,623		8,619
U.S. Treasury Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments			80,445
Government Agency Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments			17,022
Corporate Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments			17,339
(Level 1) Quoted Prices in Active Markets for Identical Assets [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets fair value disclosure	216,835		264,833
(Level 1) Quoted Prices in Active Markets for Identical Assets [Member] | Money Market Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	204,838		169,263
Restricted cash	3,374		6,506
Other assets	8,623		8,619
(Level 1) Quoted Prices in Active Markets for Identical Assets [Member] | U.S. Treasury Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments			80,445
(Level 1) Quoted Prices in Active Markets for Identical Assets [Member] | Government Agency Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments			0
(Level 1) Quoted Prices in Active Markets for Identical Assets [Member] | Corporate Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments			0
(Level 2) Significant Other Observable Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets fair value disclosure	0		34,361
(Level 2) Significant Other Observable Inputs [Member] | Money Market Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	0		0
Restricted cash	0		0
Other assets	0		0
(Level 2) Significant Other Observable Inputs [Member] | U.S. Treasury Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments			0
(Level 2) Significant Other Observable Inputs [Member] | Government Agency Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments			17,022
(Level 2) Significant Other Observable Inputs [Member] | Corporate Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments			17,339
(Level 3) Significant Unobservable Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets fair value disclosure	0		0
(Level 3) Significant Unobservable Inputs [Member] | Money Market Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	0		0
Restricted cash	0		0
Other assets	0		0
(Level 3) Significant Unobservable Inputs [Member] | U.S. Treasury Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments			0
(Level 3) Significant Unobservable Inputs [Member] | Government Agency Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments			0
(Level 3) Significant Unobservable Inputs [Member] | Corporate Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments			$ 0