Cash and Cash Equivalents and Investments (Details 1) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents
Fair Value
Amortized Cost	$ 41,960	$ 137,266
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	41,960	137,266
Certificates of Deposit
Fair Value
Amortized Cost	552	552
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	552	552
U.S. Treasury Securities
Fair Value
Amortized Cost	272,562	201,584
Gross Unrealized Gains	50	3
Gross Unrealized Losses	(27)	(66)
Estimated Fair Value	272,585	201,521
Total Short-term Investments
Fair Value
Amortized Cost	273,114	202,136
Gross Unrealized Gains	50	3
Gross Unrealized Losses	(27)	(66)
Estimated Fair Value	273,137	202,073
Total Cash and Cash Equivalents and Investments
Fair Value
Amortized Cost	315,074	339,402
Gross Unrealized Gains	50	3
Gross Unrealized Losses	(27)	(66)
Estimated Fair Value	$ 315,097	$ 339,339

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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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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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

+ References

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

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