Financial Instruments - Fair Value and Gross Unrealized Losses Related to Individual Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 33,533	$ 25,866
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	(37)	(105)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Fair Value	20,732	13,230
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Gross Unrealized Losses	(89)	(51)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	54,265	39,096
Available-for-sale Securities, Continuous Unrealized Loss Position, Gross Unrealized Losses	(126)	(156)
Corporate bonds and notes
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	12,431	14,678
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	(32)	(44)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Fair Value	14,788	13,230
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Gross Unrealized Losses	(33)	(51)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	27,219	27,908
Available-for-sale Securities, Continuous Unrealized Loss Position, Gross Unrealized Losses	(65)	(95)
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	21,102	5,216
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	(5)	(4)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Gross Unrealized Losses	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	21,102	5,216
Available-for-sale Securities, Continuous Unrealized Loss Position, Gross Unrealized Losses	(5)	(4)
Treasury and agency notes and bills
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	0	5,972
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	0	(57)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Fair Value	5,944	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Gross Unrealized Losses	(56)	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	5,944	5,972
Available-for-sale Securities, Continuous Unrealized Loss Position, Gross Unrealized Losses	$ (56)	$ (57)