Collaboration and License Agreements - Sanofi, Antibody Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2015
Disaggregation of Revenue [Line Items]
Research and development		$ 3,036,600,000	$ 2,186,100,000	$ 2,075,100,000
Treasury stock, shares acquired (in shares)		722,596
Cost of treasury stock shares received		$ 254,000,000.0
Sanofi
Disaggregation of Revenue [Line Items]
Common stock, shares issued (in shares)	23,350,365	23,350,365
Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Proceeds from collaborators				85,000,000.0
Annual funding maximum of research activities per amended agreement				130,000,000.0
Percentage of Trial Costs borne by collaborating party		80.00%
Percentage of Trial Costs borne by entity		20.00%
Percentage of repayment of development balance out of profits		50.00%
Percentage of trial costs to be reimbursed due to profitability of agreement		30.00%
Excess share of profit not required to to be applied to reimburse development cost		10.00%
Contingent reimbursement obligation	$ 2,990,000,000	$ 2,990,000,000
Treasury stock, shares acquired (in shares)		93,286
Cost of treasury stock shares received		$ 29,400,000
Starting share of profits outside the United States, based on sales, for collaborating party		65.00%
Starting share of profits outside the United States, based on sales, for Company		35.00%
Ending share of profits outside the United States, based on sales, for collaborating party		55.00%
Ending share of profits outside the United States, based on sales, for Company		45.00%
Share of losses outside the United States, for collaborating party		55.00%
Share of losses outside the United States, for Company		45.00%
Maximum amount of sales milestone payments if total sales achieve specific levels		$ 250,000,000.0
Levels of twelve month sales at which sales milestone payments would be received		$ 1,000,000,000.0
Period for achieving sales target for milestone payment, rolling basis		12 months
Period of notice to opt out of further development and or commercialization		12 months
Sanofi Collaboration Agreement, Immuno-oncology
Disaggregation of Revenue [Line Items]
Excess share of profit not required to to be applied to reimburse development cost					10.00%
Contingent reimbursement obligation	75,000,000	$ 75,000,000
Treasury stock, shares acquired (in shares)		210,733	215,387
Cost of treasury stock shares received		$ 73,300,000	$ 75,800,000
Praluent, Kevzara, and Dupixent | Sanofi Collaboration Agreement, Antibody
Disaggregation of Revenue [Line Items]
Research and development		$ 46,000,000.0	$ 47,700,000	$ 91,800,000
Employee severance costs | Sanofi
Disaggregation of Revenue [Line Items]
Restructuring-related costs	$ 35,200,000