Cash and Cash Equivalents and Investments (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents
Realized Investment Gains (Losses)	$ 0	$ 0	$ 0
Cash
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	46,600	202,989
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	46,600	202,989
US Treasury Securities
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	227,911	313,105
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	1	2
Available-for-sale Securities, Gross Unrealized Losses	(107)	(219)
Available-for-sale Securities, Current	227,805	312,888
Corporate Debt Securities [Member]
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	72,188	5,477
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	1	0
Available-for-sale Securities, Gross Unrealized Losses	(90)	(2)
Available-for-sale Securities, Current	72,099	5,475
Short-term Investments
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	300,099	318,582
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	2	2
Available-for-sale Securities, Gross Unrealized Losses	(197)	(221)
Available-for-sale Securities, Current	299,904	318,363
Cash and Cash Equivalents and Investments
Cash and Cash Equivalents
Available-for-sale Securities, Amortized Cost Basis	346,699	521,571
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	2	2
Available-for-sale Securities, Gross Unrealized Losses	(197)	(221)
Available-for-sale Securities, Current	$ 346,504	$ 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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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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