Marketable Securities (Marketable securities classified as available-for-sale) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 135,786	$ 12,981
Gross Unrealized Gains	8	616
Gross Unrealized Losses	(73)	0
Fair Value	135,721us-gaap_AvailableForSaleSecurities	13,597us-gaap_AvailableForSaleSecurities
Auction Rate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	1,175
Gross Unrealized Gains	0	615
Gross Unrealized Losses	0	0
Fair Value	0	1,790
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	46,660	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(36)	0
Fair Value	46,624	0
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	89,126	11,806
Gross Unrealized Gains	8	1
Gross Unrealized Losses	(37)	0
Fair Value	$ 89,097	$ 11,807

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

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-Name Accounting Standards Codification

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