Cash and Cash Equivalents and Investments (Details 1) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Cash and Cash Equivalents
Fair Value
Amortized Cost	$ 38,410	$ 202,989
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	38,410	202,989
U.S. Treasury Securities
Fair Value
Amortized Cost	379,307	313,105
Gross Unrealized Gains	333	2
Gross Unrealized Losses	0	(219)
Estimated Fair Value	379,640	312,888
Corporate Debt Securities
Fair Value
Amortized Cost	11,312	5,477
Gross Unrealized Gains	6	0
Gross Unrealized Losses	(3)	(2)
Estimated Fair Value	11,315	5,475
Total Short-term Investments
Fair Value
Amortized Cost	390,619	318,582
Gross Unrealized Gains	339	2
Gross Unrealized Losses	(3)	(221)
Estimated Fair Value	390,955	318,363
Total Cash and Cash Equivalents and Investments
Fair Value
Amortized Cost	429,029	521,571
Gross Unrealized Gains	339	2
Gross Unrealized Losses	(3)	(221)
Estimated Fair Value	$ 429,365	$ 521,352

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=73527792&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=73527792&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)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26626-111562

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