Collaboration Agreements - Sanofi, Antibody Narrative (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Research and development	$ 641.8	$ 498.6
Cost of Treasury stock shares received	$ 54.0
Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Percentage of Trial Costs borne by collaborating party	80.00%
Percentage of Trial Costs borne by entity	20.00%
Treasury stock, shares acquired	24,143
Cost of Treasury stock shares received	$ 10.0
Maximum amount of sales milestone payments if total sales achieve specific levels	250.0
Levels of twelve month sales at which sales milestone payments would be received	$ 1,000.0
Period for achieving sales target for milestone payment, rolling basis	12 months
Praluent, Kevzara, and Dupixent \| Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Research and development	$ 9.3	$ 13.9

Aggregate amount of twelve month sales upon which sales milestone would be received.

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The maximum amount of sales milestone payments the Company is eligible to receive when and if total annual sales achieve certain specified levels.

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The percentage of subsequent Shared Phase 3 Trial costs borne by the collaborating party upon receipt of the first positive Phase 3 trial results for a co-developed drug candidate.

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The percentage of subsequent Shared Phase 3 Trial costs borne by the Company upon receipt of the first positive Phase 3 trial results for a co-developed drug candidate.

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Period for Achieving Sales Target for Milestone Payment, Rolling Basis

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The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

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Number of shares that have been repurchased during the period and are being held in treasury.

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Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

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