Fair Value of Financial Instruments - Cash and Cash Equivalents, Available-for-sale Securities Reconciliation (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	$ 394,805	$ 356,162
Unrealized Gains	4	1
Unrealized Losses	(919)	(525)
Fair Value	393,890	355,638
Cash and cash equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	93,177	86,591
Unrealized Gains	0	0
Unrealized Losses	(3)	0
Fair Value	93,174	86,591
Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	235,775	202,106
Unrealized Gains	1	1
Unrealized Losses	(423)	(284)
Fair Value	235,353	201,823
Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	65,853	67,465
Unrealized Gains	3	0
Unrealized Losses	(493)	(241)
Fair Value	65,363	67,224
Cash \| Cash and cash equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	78,685	86,500
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	78,685	86,500
Money market funds \| Cash and cash equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	208	91
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	208	91
Commercial paper \| Cash and cash equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	14,284
Unrealized Gains	0
Unrealized Losses	(3)
Fair Value	14,281
Commercial paper \| Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	11,866	12,623
Unrealized Gains	0	0
Unrealized Losses	(7)	(3)
Fair Value	11,859	12,620
Corporate bonds \| Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	39,296	38,425
Unrealized Gains	1	1
Unrealized Losses	(139)	(64)
Fair Value	39,158	38,362
Corporate bonds \| Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	32,097	38,198
Unrealized Gains	1	0
Unrealized Losses	(388)	(175)
Fair Value	31,710	38,023
Asset backed securities \| Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	14,665	4,998
Unrealized Gains	0	0
Unrealized Losses	(32)	(12)
Fair Value	14,633	4,986
US government agencies \| Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	184,613	151,058
Unrealized Gains	0	0
Unrealized Losses	(277)	(217)
Fair Value	184,336	150,841
US government agencies \| Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	19,091	24,269
Unrealized Gains	2	0
Unrealized Losses	(73)	(54)
Fair Value	$ 19,020	$ 24,215

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=82897076&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=82897076&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=75030427&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=82896770&loc=d3e26626-111562

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