Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	$ 203,965	$ 228,198
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	65,946	30,375
Net current assets and current liabilities, fair value	65,946	30,375
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs, (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	138,019	197,823
Net current assets and current liabilities, fair value	138,058	197,880
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	0	0
Net current assets and current liabilities, fair value	0	0
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	0	0
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs, (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	9,996	9,998
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	0	0
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	9,074	26,456
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs, (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	0	0
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	0	0
US Treasury Securities [Member] | Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	56,872	3,919
US Treasury Securities [Member] | Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs, (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	0	0
US Treasury Securities [Member] | Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	0	0
Securities and obligations of U.S. government agencies | Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	0	0
Securities and obligations of U.S. government agencies | Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs, (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	128,023	187,825
Securities and obligations of U.S. government agencies | Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	0	0
Total | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	203,965	228,198
Net current assets and current liabilities, fair value	204,004	228,255
Total | Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	9,996	9,998
Total | Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	9,074	26,456
Total | US Treasury Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	56,872	3,919
Total | Securities and obligations of U.S. government agencies | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and available-for-sale investments, estimated fair value	128,023	187,825
Designated as Hedging Instrument [Member] | Foreign Exchange Forward [Member] | Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency derivatives	0	0
Foreign currency derivatives	0	0
Designated as Hedging Instrument [Member] | Foreign Exchange Forward [Member] | Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs, (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency derivatives	43	61
Foreign currency derivatives	(4)	(4)
Designated as Hedging Instrument [Member] | Foreign Exchange Forward [Member] | Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency derivatives	0	0
Foreign currency derivatives	0	0
Designated as Hedging Instrument [Member] | Total | Foreign Exchange Forward [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency derivatives	43	61
Foreign currency derivatives	$ (4)	$ (4)