Fair Value of Financial Instruments - Cash and Cash Equivalents, Available-for-sale Securities Reconciliation (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	$ 343,204	$ 289,739
Unrealized Gains	43	7
Unrealized Losses	(162)	(165)
Fair Value	343,085	289,581
Cash and cash equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	90,517	86,737
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	90,517	86,737
Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	212,342	157,167
Unrealized Gains	10	5
Unrealized Losses	(142)	(53)
Fair Value	212,210	157,119
Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	40,345	45,835
Unrealized Gains	33	2
Unrealized Losses	(20)	(112)
Fair Value	40,358	45,725
Cash \| Cash and cash equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	90,472	72,673
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	90,472	72,673
Money market funds \| Cash and cash equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	45	473
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	45	473
Commercial paper \| Cash and cash equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost		13,591
Unrealized Gains		0
Unrealized Losses		0
Fair Value		13,591
Commercial paper \| Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	20,930	14,782
Unrealized Gains	4	5
Unrealized Losses	(1)	0
Fair Value	20,933	14,787
Corporate bonds \| Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	31,786	13,490
Unrealized Gains	3	0
Unrealized Losses	(22)	(11)
Fair Value	31,767	13,479
Corporate bonds \| Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	37,490	11,243
Unrealized Gains	32	0
Unrealized Losses	(20)	(41)
Fair Value	37,502	11,202
Asset backed securities \| Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	491	1,235
Unrealized Gains	0
Unrealized Losses	0
Fair Value	491	1,235
Asset backed securities \| Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	2,855	5,091
Unrealized Gains	1	2
Unrealized Losses	0	0
Fair Value	2,856	5,093
US government agencies \| Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost	159,135	127,660
Unrealized Gains	3
Unrealized Losses	(119)	(42)
Fair Value	$ 159,019	127,618
US government agencies \| Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized Cost		29,501
Unrealized Gains		0
Unrealized Losses		(71)
Fair Value		$ 29,430

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)
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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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