Balance Sheet Account Detail (Held to Maturity Marketable Securities) (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		$ 20,988,000
Gross Unrealized Gain		2,000
Gross Unrealized Loss		(21,000)
Fair Value		20,969,000
Unrealized gains (losses) on investments	$ 0
Agency bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		6,488,000
Gross Unrealized Gain		2,000
Gross Unrealized Loss		0
Fair Value		6,490,000
Corporate bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		10,513,000
Gross Unrealized Gain		0
Gross Unrealized Loss		(21,000)
Fair Value		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