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Financial Instruments and Fair Value Measurements - Composition of Marketable Securities (Details) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2016
Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities, Cost   $ 278,290
Available-for-sale Securities, Gross Unrealized Gains, Before Tax   5
Available-for-sale Securities, Gross Unrealized Losses, Before Tax   (540)
Available-for-sale Securities, Fair Value   277,755
Short-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities, Cost   165,629
Available-for-sale Securities, Gross Unrealized Gains, Before Tax   5
Available-for-sale Securities, Gross Unrealized Losses, Before Tax   (211)
Available-for-sale Securities, Fair Value   165,423
Long-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities, Cost   112,661
Available-for-sale Securities, Gross Unrealized Losses, Before Tax   (329)
Available-for-sale Securities, Fair Value   112,332
Commercial paper [Member] | Short-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities, Cost   21,991
Available-for-sale Securities, Fair Value   $ 21,991
Commercial paper [Member] | Maximum [Member] | Short-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities Contractual Maturity 1 year 1 year
Certificates of deposit [Member] | Short-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities, Cost   $ 6,615
Available-for-sale Securities, Fair Value   6,615
Certificates of deposit [Member] | Long-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities, Cost   12,392
Available-for-sale Securities, Fair Value   $ 12,392
Certificates of deposit [Member] | Maximum [Member] | Short-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities Contractual Maturity   1 year
Certificates of deposit [Member] | Maximum [Member] | Long-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities Contractual Maturity   2 years
Certificates of deposit [Member] | Minimum [Member] | Long-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities Contractual Maturity   1 year
Corporate notes [Member] | Short-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities, Cost   $ 108,739
Available-for-sale Securities, Gross Unrealized Gains, Before Tax   5
Available-for-sale Securities, Gross Unrealized Losses, Before Tax   (173)
Available-for-sale Securities, Fair Value   108,571
Corporate notes [Member] | Long-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities, Cost   43,857
Available-for-sale Securities, Gross Unrealized Losses, Before Tax   (109)
Available-for-sale Securities, Fair Value   $ 43,748
Corporate notes [Member] | Maximum [Member] | Short-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities Contractual Maturity 1 year 1 year
Corporate notes [Member] | Maximum [Member] | Long-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities Contractual Maturity 2 years 2 years
Corporate notes [Member] | Minimum [Member] | Long-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities Contractual Maturity 1 year 1 year
Securities of government-sponsored entities [Member] | Short-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities, Cost   $ 28,284
Available-for-sale Securities, Gross Unrealized Losses, Before Tax   (38)
Available-for-sale Securities, Fair Value   28,246
Securities of government-sponsored entities [Member] | Long-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities, Cost   56,412
Available-for-sale Securities, Gross Unrealized Losses, Before Tax   (220)
Available-for-sale Securities, Fair Value   $ 56,192
Securities of government-sponsored entities [Member] | Maximum [Member] | Short-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities Contractual Maturity 1 year 1 year
Securities of government-sponsored entities [Member] | Maximum [Member] | Long-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities Contractual Maturity 2 years 2 years
Securities of government-sponsored entities [Member] | Minimum [Member] | Long-term Investments [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Available-for-sale Securities Contractual Maturity 1 year 1 year