Financial Instruments - Fair Value and Gross Unrealized Losses Related to Individual Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	$ 24,340	$ 33,201
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	(325)	(191)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Fair Value	15,109	20,691
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Gross Unrealized Losses	(131)	(112)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	39,449	53,892
Available-for-sale Securities, Continuous Unrealized Loss Position, Gross Unrealized Losses	(456)	(303)
Corporate bonds and notes
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	24,340	30,811
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	(325)	(189)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Fair Value	9,188	14,762
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Gross Unrealized Losses	(52)	(41)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	33,528	45,573
Available-for-sale Securities, Continuous Unrealized Loss Position, Gross Unrealized Losses	(377)	(230)
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value		2,390
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses		(2)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Fair Value		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Gross Unrealized Losses		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		2,390
Available-for-sale Securities, Continuous Unrealized Loss Position, Gross Unrealized Losses		(2)
Treasury and agency notes and bills
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Fair Value	5,921	5,929
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or more, Gross Unrealized Losses	(79)	(71)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	5,921	5,929
Available-for-sale Securities, Continuous Unrealized Loss Position, Gross Unrealized Losses	$ (79)	$ (71)