Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Millions		1 Months Ended							3 Months Ended							6 Months Ended	9 Months Ended
	Jul. 02, 2020	Dec. 31, 2019	Sep. 30, 2019	Apr. 30, 2019	Mar. 31, 2019	Jan. 31, 2019	Nov. 30, 2018	Oct. 31, 2018	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Oct. 27, 2020
Other Commitments [Line Items]
Asset Acquisition, Contingent Consideration, Milestone Payments																	$ 160.7
Research and development									$ 285.9			$ 232.9					707.9	$ 616.4
Lonza Group AG
Other Commitments [Line Items]
Remaining total commitments									1,432.3							$ 1,432.3	1,432.3
Unrecorded Unconditional Purchase Obligation, Undiscounted Fixed Payment Amount									105.5							105.5	105.5
Other Third Party Manufacturers
Other Commitments [Line Items]
Remaining total commitments									78.6							78.6	78.6
Collaborative Arrangement | Caelum
Other Commitments [Line Items]
Collaboration Agreement, Upfront Payment						$ 30.0
Research and development									3.6			0.2					6.0	$ 0.5
Collaboration Agreement, Potential Future Payments, Milestone Achievement						30.0			15.0
Derivative Liability		$ 27.1									$ 27.1
Collaboration Agreement, Acquisition Option, Maximum Potential Future Payment						500.0
Collaboration Agreement, Milestone Payment										$ 15.0						30.0
Collaboration Agreement, Potential Future Payments											20.0
Collaboration Agreement, Payment For Additional Equity Interest		$ 60.0
Zealand | Zealand
Other Commitments [Line Items]
Payments To Acquire Licenses And Equity Investment					$ 40.0
Prepaid Research And Development Expense					5.0								$ 5.0
Research and development													$ 21.2
Collaboration Agreement, Potential Future Payments, Milestone Achievement																	610.0
Collaboration Agreement, Potential Future Payments, Additional Milestone Achievement																	115.0
Collaboration Agreement, Potential Future Payments, Option Fee																	15.0
Affibody AB | Affibody
Other Commitments [Line Items]
Payments To Acquire Licenses And Equity Investment				$ 25.0
Affibody AB | Eidos
Other Commitments [Line Items]
Payments To Acquire Licenses And Equity Investment			$ 50.0
Research and development												$ 30.1
Collaboration Agreement, Potential Future Payments, Milestone Achievement																	30.0
Dicerna Pharmaceutical Collaboration Agreement | Dicerna
Other Commitments [Line Items]
Payments To Acquire Licenses And Equity Investment								$ 37.0
Research and development											$ 20.0			$ 26.7
Research and Development Arrangement, Potential Payment, Maximum									604.1							604.1	604.1
License Agreement 1 | Halozyme Therapeutics, Inc
Other Commitments [Line Items]
Research and Development Arrangement, Potential Payment, Maximum									155.0							155.0	155.0
Research and development															$ 40.0
Research and Development Agreement, Option Fee									8.0							8.0	8.0
Research and Development Arrangement, Potential Contingent Payment, Maximum									160.0							160.0	160.0
Collaboration and License Agreement
Other Commitments [Line Items]
Collaboration Agreement, Potential Future Payments, Milestone Achievement																	114.0
Eidos
Other Commitments [Line Items]
Payments to Acquire Equity Method Investments			$ 19.9
Caelum | Collaborative Arrangement
Other Commitments [Line Items]
Equity Method Investment, Aggregate Cost						$ 57.1
Zealand
Other Commitments [Line Items]
Payments to Acquire Equity Method Investments					$ 13.8
Dicerna
Other Commitments [Line Items]
Payments to Acquire Equity Method Investments								$ 10.3
SEC
Other Commitments [Line Items]
Litigation Settlement, Amount Awarded to Other Party	$ 21.5
DOJ And OIG
Other Commitments [Line Items]
Loss Contingency Accrual, Payments				$ 13.1
Canadian Patented Medicine Prices Review Board
Other Commitments [Line Items]
Loss Contingency, Estimate of Possible Loss									42.2							$ 42.2	$ 42.2
Syntimmune, Inc
Other Commitments [Line Items]
Asset Acquisition, Contingent Consideration, Milestone Payments							$ 800.0
Syntimmune, Inc | Subcutaneous Formulation
Other Commitments [Line Items]
Asset Acquisition, Contingent Consideration, Milestone Payments							$ 130.0
Astellas Pharam [Member]
Other Commitments [Line Items]
Asset Acquisition, Contingent Consideration, Milestone Payments									$ 71.5
Subsequent Event | Canadian Patented Medicine Prices Review Board
Other Commitments [Line Items]
Escrow Deposit																			$ 61.5