Balance Sheet Account Detail (Held to Maturity Marketable Securities) (Details)	3 Months Ended	6 Months Ended
	Jun. 30, 2017 USD ($) security	Jun. 30, 2017 USD ($) security	Dec. 31, 2016 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 10,000,000	$ 10,000,000	$ 20,988,000
Gross Unrealized Gain	0	0	2,000
Gross Unrealized Loss	(3,000)	(3,000)	(21,000)
Fair Value	$ 9,997,000	$ 9,997,000	20,969,000
Held-to-maturity debt securities held | security	4	4
Time held in unrealized loss position (less than)		11 months
Unrealized gains (losses) on investments	$ 0	$ 0
Average maturity		1 month
Agency bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,999,000	$ 3,999,000	6,488,000
Gross Unrealized Gain	0	0	2,000
Gross Unrealized Loss	(1,000)	(1,000)	0
Fair Value	3,998,000	3,998,000	6,490,000
Corporate bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	6,001,000	6,001,000	10,513,000
Gross Unrealized Gain	0	0	0
Gross Unrealized Loss	(2,000)	(2,000)	(21,000)
Fair Value	$ 5,999,000	$ 5,999,000	10,492,000
Commercial paper
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost			3,987,000
Gross Unrealized Gain			0
Gross Unrealized Loss			0
Fair Value			$ 3,987,000