Financial Instruments - Fair Value and Gross Unrealized Losses Related to Individual Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	$ 33,201	$ 25,866
Gross Unrealized Losses, Less Than 12 Months	(191)	(105)
Fair Value, 12 Months Or More	20,691	13,230
Gross Unrealized Losses, 12 Months or More	(112)	(51)
Fair Value, Total	53,892	39,096
Gross Unrealized Losses, Total	(303)	(156)
Corporate bonds and notes
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	30,811	14,678
Gross Unrealized Losses, Less Than 12 Months	(189)	(44)
Fair Value, 12 Months Or More	14,762	13,230
Gross Unrealized Losses, 12 Months or More	(41)	(51)
Fair Value, Total	45,573	27,908
Gross Unrealized Losses, Total	(230)	(95)
Treasury and agency notes and bills
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	0	5,972
Gross Unrealized Losses, Less Than 12 Months	0	(57)
Fair Value, 12 Months Or More	5,929	0
Gross Unrealized Losses, 12 Months or More	(71)	0
Fair Value, Total	5,929	5,972
Gross Unrealized Losses, Total	(71)	(57)
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	2,390	5,216
Gross Unrealized Losses, Less Than 12 Months	(2)	(4)
Gross Unrealized Losses, 12 Months or More	0	0
Fair Value, Total	2,390	5,216
Gross Unrealized Losses, Total	$ (2)	$ (4)