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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2014
Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments $ 649,037 $ 523,607
Commercial Paper [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 20,488 124,319
Corporate notes and bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 548,370 399,288
Certificates of Deposit [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 20,099  
US Government Agencies Debt Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 60,080  
Level 1 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 191,251 104,581
Level 2 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 692,872 785,329
Total [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total 884,123 889,910
Cash [Member] | Level 1 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 110,110 69,333
Cash [Member] | Level 2 [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 0 0
Cash [Member] | Total [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 110,110 69,333
Cash Equivalents [Member] | Level 1 [Member] | Money Market Funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 81,141 35,248
Cash Equivalents [Member] | Level 1 [Member] | Commercial Paper [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 0 0
Cash Equivalents [Member] | Level 2 [Member] | Money Market Funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 0 0
Cash Equivalents [Member] | Level 2 [Member] | Commercial Paper [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 43,835 261,722
Cash Equivalents [Member] | Total [Member] | Money Market Funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 81,141 35,248
Cash Equivalents [Member] | Total [Member] | Commercial Paper [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 43,835 261,722
Short-term Investments [Member] | Level 1 [Member] | Commercial Paper [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Short-term Investments [Member] | Level 1 [Member] | Corporate notes and bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Short-term Investments [Member] | Level 1 [Member] | Certificates of Deposit [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0  
Short-term Investments [Member] | Level 1 [Member] | US Government Agencies Debt Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0  
Short-term Investments [Member] | Level 2 [Member] | Commercial Paper [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 20,488 124,319
Short-term Investments [Member] | Level 2 [Member] | Corporate notes and bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 359,875 143,932
Short-term Investments [Member] | Level 2 [Member] | Certificates of Deposit [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 16,604  
Short-term Investments [Member] | Level 2 [Member] | US Government Agencies Debt Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 14,019  
Short-term Investments [Member] | Total [Member] | Commercial Paper [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 20,488 124,319
Short-term Investments [Member] | Total [Member] | Corporate notes and bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 359,875 143,932
Short-term Investments [Member] | Total [Member] | Certificates of Deposit [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 16,604  
Short-term Investments [Member] | Total [Member] | US Government Agencies Debt Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 14,019  
Other Long-term Investments [Member] | Level 1 [Member] | Corporate notes and bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0 0
Other Long-term Investments [Member] | Level 1 [Member] | Certificates of Deposit [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0  
Other Long-term Investments [Member] | Level 1 [Member] | US Government Agencies Debt Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 0  
Other Long-term Investments [Member] | Level 2 [Member] | Corporate notes and bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 188,495 255,356
Other Long-term Investments [Member] | Level 2 [Member] | Certificates of Deposit [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 3,495  
Other Long-term Investments [Member] | Level 2 [Member] | US Government Agencies Debt Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 46,061  
Other Long-term Investments [Member] | Total [Member] | Corporate notes and bonds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 188,495 255,356
Other Long-term Investments [Member] | Total [Member] | Certificates of Deposit [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments 3,495  
Other Long-term Investments [Member] | Total [Member] | US Government Agencies Debt Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments $ 46,061