Balance Sheet Account Detail (Held-to Maturity Marketable Securities) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($)
Marketable Securities [Abstract]
Amortized Cost	$ 20,988	$ 52,768
Gross Unrealized Gain	2	1
Gross Unrealized Loss	(21)	(53)
Fair Value	$ 20,969	52,716
Average maturity	5 months
Agency bonds
Marketable Securities [Abstract]
Amortized Cost	$ 6,488	8,000
Gross Unrealized Gain	2	0
Gross Unrealized Loss	0	(20)
Fair Value	6,490	7,980
Corporate bonds
Marketable Securities [Abstract]
Amortized Cost	10,513
Gross Unrealized Gain	0
Gross Unrealized Loss	(21)
Fair Value	10,492
Government securities
Marketable Securities [Abstract]
Amortized Cost		40,824
Gross Unrealized Gain		1
Gross Unrealized Loss		(33)
Fair Value		40,792
Commercial paper
Marketable Securities [Abstract]
Amortized Cost	3,987	3,944
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fair Value	3,987	$ 3,944
Debt securities
Marketable Securities [Abstract]
Fair Value	$ 10,500
Held to maturity securities, number of securities held | security	5
Duration in loss position	5 months