The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274

Name:	us-gaap_AmortizationOfIntangibleAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Intangible Assets - Narrative (Details) $ in Thousands, € in Millions	1 Months Ended		3 Months Ended	9 Months Ended
	Apr. 30, 2017 USD ($)	Apr. 30, 2017 EUR (€)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 EUR (€)	Sep. 30, 2017 USD ($)
Finite-Lived Intangible Assets [Line Items]
Milestone payments that triggered intangible asset recognition				$ 0		$ 8,712
Amortization expense			$ 200	600
Estimated future amortization expense for intangible assets, remainder of year			200	200
Estimated future amortization expense for intangible assets, per year thereafter			$ 800	800
Collaborative Arrangement \| Ipsen
Finite-Lived Intangible Assets [Line Items]
Milestone payments that triggered intangible asset recognition	$ 8,700	€ 8.0		$ 8,700	€ 8.0