Legal Matters - Narrative (Details) € in Thousands, $ in Millions	Dec. 20, 2018 USD ($) shares	Sep. 26, 2016 EUR (€)
Regeneron v.s. Merus N.V. \| Settled litigation
Loss Contingencies [Line Items]
Common stock purchased (in shares) \| shares	600,000
Purchase price of common stock \| $	$ 15.0
Amgen v.s. Regeneron \| Pending litigation
Loss Contingencies [Line Items]
Loss Contingency, damages sought, per unit produced		€ 10
Loss contingency, damages sought		€ 10,000

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14557-108349

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14435-108349

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration