Financial Instruments - Fair Value and Gross Unrealized Losses Related to Individual Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 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	$ 37,655	$ 25,866
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	(102)	(105)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Fair Value	3,811	13,230
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Gross Unrealized Losses	(21)	(51)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	41,466	39,096
Available-for-sale Securities, Continuous Unrealized Loss Position, Gross Unrealized Losses	(123)	(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	27,962	14,678
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	(62)	(44)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Fair Value	3,811	13,230
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Gross Unrealized Losses	(21)	(51)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	31,773	27,908
Available-for-sale Securities, Continuous Unrealized Loss Position, Gross Unrealized Losses	(83)	(95)
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	5,962	5,972
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	(38)	(57)
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	5,962	5,972
Available-for-sale Securities, Continuous Unrealized Loss Position, Gross Unrealized Losses	(38)	(57)
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	3,731	5,216
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	(2)	(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	3,731	5,216
Available-for-sale Securities, Continuous Unrealized Loss Position, Gross Unrealized Losses	$ (2)	$ (4)