Cash and Cash Equivalents and Investments (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash and Cash Equivalents
Realized Investment Gains (Losses)	$ 0	$ 0	$ 0
Cash
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	202,989	137,266
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	0	0
Available-for-sale Securities, Gross Unrealized Losses	0	0
Available-for-sale Securities, Current	202,989	137,266
Certificates of Deposit
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis		552
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments		0
Available-for-sale Securities, Gross Unrealized Losses		0
Available-for-sale Securities, Current		552
US Treasury Securities
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	313,105	201,584
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	2	3
Available-for-sale Securities, Gross Unrealized Losses	(219)	(66)
Available-for-sale Securities, Current	312,888	201,521
Corporate Debt Securities [Member]
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	5,477
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	0
Available-for-sale Securities, Gross Unrealized Losses	(2)
Available-for-sale Securities, Current	5,475
Short-term Investments
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	318,582	202,136
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	2	3
Available-for-sale Securities, Gross Unrealized Losses	(221)	(66)
Available-for-sale Securities, Current	318,363	202,073
Cash and Cash Equivalents and Investments
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	521,571	339,402
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	2	3
Available-for-sale Securities, Gross Unrealized Losses	(221)	(66)
Available-for-sale Securities, Current	$ 521,352	$ 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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+ 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=51813488&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=49167202&loc=d3e26626-111562

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No definition available.

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The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.3(a))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 325
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6485959&loc=d3e22529-158502

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