Marketable Securities (Marketable securities classified as available-for-sale) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 112,705	$ 135,786
Gross Unrealized Gains	6	8
Gross Unrealized Losses	(26)	(73)
Fair Value	112,685us-gaap_AvailableForSaleSecurities	135,721us-gaap_AvailableForSaleSecurities
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	46,648	46,660
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(9)	(36)
Fair Value	46,639	46,624
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	66,057	89,126
Gross Unrealized Gains	6	8
Gross Unrealized Losses	(17)	(37)
Fair Value	$ 66,046	$ 89,097

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)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

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

-Publisher FASB

-Name Accounting Standards Codification

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-Section 50

-Paragraph 5

-Subparagraph (aa)

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

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- Definition

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

-Topic 320

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

+ References

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-Name Accounting Standards Codification

-Topic 320

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