Balance Sheet Account Detail (Held-to Maturity Marketable Securities) (Details) (USD $)	Dec. 31, 2014 securities	Dec. 31, 2013
Marketable Securities [Abstract]
Amortized Cost	$ 59,871,000	$ 31,313,000
Gross Unrealized Gain	(5,000)	(3,000)
Gross Unrealized Loss	(8,000)	(3,000)
Fair Value	59,868,000	31,313,000
Held to Maturity Securities, Number of Securities Held	16
Fair value of investments, Unrealized Loss Position	18,600,000
Asset backed securities
Marketable Securities [Abstract]
Amortized Cost	3,633,000	1,405,000
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fair Value	3,633,000	1,405,000
Corporate bonds
Marketable Securities [Abstract]
Amortized Cost	15,707,000
Gross Unrealized Gain	0
Gross Unrealized Loss	(8,000)
Fair Value	15,699,000
Commercial paper
Marketable Securities [Abstract]
Amortized Cost	40,531,000	29,908,000
Gross Unrealized Gain	(5,000)	(3,000)
Gross Unrealized Loss	0	(3,000)
Fair Value	$ 40,536,000	$ 29,908,000

Held to Maturity Securities, Number of Securities Held

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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

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Amount before other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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This item represents the fair value of securities categorized as held-to-maturity that have been in a continuous unrealized loss position for less than twelve months.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

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Fair value of investments in debt securities classified as held-to-maturity.

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