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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2012
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments $ 523,607 $ 231,784
Commercial paper
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments 124,319 72,835
Corporate notes and bonds securities
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments 399,288 157,948
US government agencies securities
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments   1,001
Level 1
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Total 104,581 82,907
Level 2
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Total 785,329 231,784
Level 3
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Total 0 0
Total
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Total 889,910 314,691
Cash [Member] | Level 1
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Cash and cash equivalents 69,333 47,478
Cash [Member] | Level 2
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Cash and cash equivalents 0 0
Cash [Member] | Level 3
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Cash and cash equivalents 0 0
Cash [Member] | Total
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Cash and cash equivalents 69,333 47,478
Cash Equivalents [Member] | Level 1 | Money market funds
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Cash and cash equivalents 35,248 35,429
Cash Equivalents [Member] | Level 1 | Commercial paper
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Cash and cash equivalents 0 0
Cash Equivalents [Member] | Level 2 | Money market funds
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Cash and cash equivalents 0 0
Cash Equivalents [Member] | Level 2 | Commercial paper
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Cash and cash equivalents 261,722 36,082
Cash Equivalents [Member] | Level 3 | Money market funds
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Cash and cash equivalents 0 0
Cash Equivalents [Member] | Level 3 | Commercial paper
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Cash and cash equivalents 0 0
Cash Equivalents [Member] | Total | Money market funds
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Cash and cash equivalents 35,248 35,429
Cash Equivalents [Member] | Total | Commercial paper
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Cash and cash equivalents 261,722 36,082
Short-term Investments [Member] | Level 1 | Commercial paper
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments 0 0
Short-term Investments [Member] | Level 1 | Corporate notes and bonds securities
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments 0 0
Short-term Investments [Member] | Level 1 | US government agencies securities
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments   0
Short-term Investments [Member] | Level 2 | Commercial paper
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments 124,319 36,753
Short-term Investments [Member] | Level 2 | Corporate notes and bonds securities
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments 143,932 157,948
Short-term Investments [Member] | Level 2 | US government agencies securities
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments   1,001
Short-term Investments [Member] | Level 3 | Commercial paper
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments 0 0
Short-term Investments [Member] | Level 3 | Corporate notes and bonds securities
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments 0 0
Short-term Investments [Member] | Level 3 | US government agencies securities
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments   0
Short-term Investments [Member] | Total | Commercial paper
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments 124,319 36,753
Short-term Investments [Member] | Total | Corporate notes and bonds securities
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments 143,932 157,948
Short-term Investments [Member] | Total | US government agencies securities
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments   1,001
Other Long-term Investments [Member] | Level 1 | Corporate notes and bonds securities
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments 0  
Other Long-term Investments [Member] | Level 2 | Corporate notes and bonds securities
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments 255,356  
Other Long-term Investments [Member] | Level 3 | Corporate notes and bonds securities
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments 0  
Other Long-term Investments [Member] | Total | Corporate notes and bonds securities
    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Investments $ 255,356